history we find that the study of it sprang out of the various attempts to solve the problems of why and how living beings come into existence.

It would be beyond the scope of this work to enter at all fully into any account of the earlier of these inquiries from those of Aristotle downwards; but it may be of some use to point out the chief steps by which in modern times embryology has been established as a distinct branch of knowledge.

From the very first, incubated bird's eggs, and especially hen's eggs, owing to their abundance at all seasons, and the ease with which they could be examined, became special objects of study. Aristotle examined the growing chick within the egg, and gave the name of punctum saliens to the 'bloody palpitating point,' which marks the growing heart in the early days of incubation. Since his time all observers have had recourse to the hen's egg; and though it may be urged that the highly specialised characters of the avian type unfit it for so general a purpose as that of serving as the foundation of embryology, the practical advantages of the bird's egg over either the mammalian or any other ovum, are so many, that it must always continue to be, as it has been, a chief object of study.

From the time of Aristotle down to that of Fabricius of Aquapendente so little progress in real observation of facts had been made, that we find the latter anatomist (De Formatione Ovi et Pulli, 1621) describing the chick as being formed out of the chalaza of the white of the egg; a view which lived long afterwards, and whose influence may still be recognized in the names 'tread' or 'treadle' which the housewife sometimes gives to those portions of thickened albumen.

Harvey was the first to clearly establish that the essential part of the hen's egg, that out of which the embryo pro

ceeded, was the cicatricula. This Fabricius had looked upon as a blemish, a scar left by a broken peduncle. In his Anatomical Exercises on the Generation of Animals (1651), Harvey describes the little cicatricula as expanding under the influence of incubation into a wider structure, which he calls the eye of the egg; and at the same time separating into a colliquamentum. In this colliquamentum, according to him, there appears, as the first rudiment of the embryo, the heart or punctum saliens, together with the blood-vessels. These gradually gather round them the solid parts of the body of the chick. Harvey clearly was of opinion that the embryo arose, by the successive formation of parts, out of the homogeneous nearly liquid colliquamentum. He was an early advocate of the doctrine of epigenesis.

Notwithstanding the weight of Harvey's authority, the doctrine of epigenesis subsequently gave way to that of evolution, according to which the embryo pre-existed, even though invisible, in the ovum, and the changes which took place during incubation consisted not in a formation of parts, but in a growth, i. e. in an expansion with concomitant changes, of the already existing germ. Of this theory Malpighi is frequently said to have been the founder. In a limited sense this is true. In his letter to the Royal Society of London, De Formatione Pulli in Ovo (1672), he confesses himself compelled to admit that even in unincubated eggs an embryo was present (Quare pulli stamina in ovo pre-existere, altioremque originem nacta esse fateri convenit). Yet he evidently struggled against such a conclusion, and instead of developing a consistent theory of evolution, left the earliest stages of the embryo as too mysterious to be profitable objects of study, and contented himself with tracing out the events of later days. From his descriptions it is clear that his so-called unincubated eggs

had under the warmth of summer already made considerable progress in development.

The man who first logically worked out a theory of evolution and became its most distinguished and zealous advocate was Haller (Sur la Formation du Cœur dans le Poulet, 1758, and Elementa Physiologia, Liber xxix. 1766).

This great anatomist insisted that the embryo existed even in the unincubated egg though in a rudimentary form, and indeed invisible. He supposed that it was a vermiform structure composed of all the essential parts of a full-grown animal in an undeveloped state, and that the effect of incubation was to educe or evolve these undeveloped organs into an adult condition. The same views were urged with characteristic extravagance by Bonnet (Considérations sur les corps organisés, 1762).

This doctrine of evolution or prædilineation, as it was called at the time, was doomed to be overthrown even in Haller's own day.

In an inaugural dissertation entitled Theoria Generationis, published 1759, Casper Frederick Wolff laid the foundations of not only modern Embryology, but modern Histology. He shewed that the cicatricula of the unincubated hen's egg consisted of a congeries of particles (such as we now call cells) all alike, or divisible into groups only, and that anything like distinct rudiments of an embryo were wholly absent. Out of these particles the embryo was built up by means of a series of successive changes (several of which he described in detail, especially in his work on the Formation of the Alimentary Canal, 1768), part being added to part, and parts once formed being modified into fresh parts. Thus the old imperfect theory of evolution was supplanted by a view, which, under the term of epigenesis, was in reality a more complete and truer theory of evolution. Wolff also shewed that all the parts as well of plants as of animals could be conceived of

as being arrangements of these particles or cells variously modified, and that all the phenomena of the form and structure of living beings were to be regarded as the results of a variable nutritive energy, to which he gave the name vis essentialis.

Haller complained of Wolff, that he had attempted to make a great leap instead of being contented with small onward steps. Wolff's leap proved too great for his time. While his insight into the fundamental doctrines of histology remained for the most part without fruit till the next century, so also the way he opened up in embryology was successfully followed by no one for many years after.

In 1816 that admirable teacher Döllinger, of Würzburg, induced Pander to take up the study of the incubated hen's egg. We owe to Pander (Dissertatio Inauguralis sistens Historiam Metamorphoseos quam Ovum Incubatum prioribus quinque diebus subit, and Beiträge zur Entwickelungsgeschichte des Hühnchens im Eie) a clear and excellent description of many of the changes which take place during the early days of incubation. It was he who introduced the term blastoderm. He too first drew attention to the distinction of the three layers, serous, mucous, and vascular. But his greatest merit perhaps consisted in the fact of his studies having been the exciting cause of those of Von Baer.

Coming to Würzburg to study under Döllinger, and finding Pander busily engaged in his embryological work, Von Baer enthusiastically took up the same subject, and thenceforward devoted the greater part of his life to it.

Of the results of his labours, which are embodied in his Entwickelungsgeschichte der Thiere, 1828, 1837, this simply may be said. Von Baer found the true line of inquiry already marked out by Wolff. He followed up that line so sedulously and with such success, that nearly all the work which has been done since his day up to the present time, in Vertebrate

Embryology, may be regarded as little more than an extension, with corrections, of his observations. Were it desirable to re-publish Von Baer's work, the corrections and expansions of matters of fact necessary to bring it up to the present time, as the phrase goes, would, with some few exceptions, be of minor importance, though they might be many. The theoretical considerations embodied in his Scholia through which he interprets the morphological significance of embryological facts are of great and lasting importance, though they need some modifications in order to bring them into harmony with the theory of natural selection. Since Von Baer's time, the advances made in Vertebrate Embryology, through the elaborate work of Remak, the labours of Rathke, Allen Thomson and others, the admirable lectures of Kölliker, and the researches of more recent inquirers, though many and varied, cannot be said to constitute any epochs in the history of the subject, such as that which was marked by Von Baer, and before him by Wolff. We may perhaps make an exception in favour of the discovery by Purkinje, of the germinal vesicle in the fowl's ovarian ovum (1825). This led to Von Baer's discovery of the mammalian ovum (1827), which first rendered possible a consistent view of mammalian generation.

The study of invertebrate embryology has, on the other hand, during the last few years produced the most striking results.

In the following pages we propose to follow in the path thus marked out by the history of the subject. We begin with the chick as being the animal which has been most studied, and the study of which is easiest, and most fruitful for the beginner. The first part accordingly will be devoted to a description of the changes undergone by an incubated hen's egg, especially during the early days of incubation. We shall endeavour to explain, with such details as are necessary,