LECTURES ON EMBRYOLOGY. 



space and condense arounJ the germinative ves- 

 icle. So that there was no more possibility of 

 doubt that the vesicle and the germinative dot did 

 exist there before the vitellus* At ether times, the 

 srertninative vesicle alone has been observed in the 

 developing eggs. There are other instances where 

 the ovarian egg presents neither gerrainative ves- 

 icle nor germinative dot during the formation of 

 the yolk. This shows that even the question of 

 the fundamental structure of the egg, in order to 

 be sofved, calls yet for minate and serial research- 

 es. 



In the interstices of the granules or little cellules 

 which compose the vitellus, is contained a transpa- 

 rent liquid more consistent than water, since it re^ 

 sists a certain pressure. When the egg is formed 

 this liquid tends towards a centre and agglomerates 

 itself there under the form of a transparent sphere, 

 the appearance of which precedes the ordinary 

 phases of the dividing of the yolk. 



Whether the progress is the result of the mix- 

 ture of the contents of the germinative vesicle and 

 the germinative dot; or the changes are intro- 

 duced simply owing to the fact that the egg has 

 arrived at its maturity ; whether it relies simply 

 apon the yolk to undergo those changes, is a point 

 which it is impossible to decide at present. Gen- 

 erally, when the yolk undergoes the first change 

 by which the germ is formed, the germinative 

 vesicle acd the germinative dot have already dis- 

 appeared ; bat in some instances^ the germinative 

 vesicle and the gerrainative dot have been ob- 

 served within the yolk, when another mass, (the 

 clear sphere) which generally appears after those 

 have gone, had been formed in another portion of 

 the egg, as represented in PI. XXV, fig. Hi so that 

 changes which have been known to be connected 

 with the first formation, changes giving rise 

 to the germ such modifications are observed in 

 the yolk when the germinative vesicle is still 

 within. 



Therefore, it cannot be absolutely said that the 

 bursting of the germinative vesicle, and the mix- 

 ture of the substance contained within it, is prop- 

 erly the cause of the changes now taking place. 

 It may have an influence upon the yolk, by which 

 those changes are accelerated or facilitated; but 

 that it is properly tfee cause, cannot be main- 

 tained. 



Well, to understand all these changes which take 

 place within the egg v they must be conceived as 

 successive modifications of substance. We know 

 that one sort of egg will only give rise to one sort 

 of animak Therefore we racist admit, that as an 

 gg of ene kind gives rise only to one sort of ani- 

 mal, there must be an immaterial principle presid- 

 ing over these changes, which is invariable in its 

 nature, and is properly the cause of tke whole 

 process, 



But now the changes which take place in the 

 yolk vary in different classes of animals. In some 

 fcbev consist <?f a division of the yolk, which is 



successively repeated and repeated, till the whole 

 mass of the yolk has been so much subdivided as 

 then to consist of innumerable little masses, aris- 

 ing from the subdivision, from the repeated subdi- 

 vision of the primitive mass into successively more 

 and more numerous parts. In others, the division 

 is only partial. On one side of the yolk there is a 

 depression formed, which does not penetrate across 

 the whole mass, and then another, which will be 

 formed at right angles with the first, thus forming 

 four partial divisions; and that being repeated, 

 the surface of the yolk, on one side of this mass 

 may be divided into little fractions, though a great 

 portion of the yolk takes no part in this process of 

 repeated division and subdivision. In many ani- 

 mals the division of the yolk is most wonderfully 

 regular. 



The dividing of the yolk is probably a general 

 phenomenon, appearing in all eggs, though obser 

 vation has not revealed it to us in all classes with 

 the same certainty. Its generality, however, is 

 difficult to trace at present; as its various modifi- 

 cations have not been reduced to one common 

 type > however, the fact is already ascertained in 

 the class of Mammalia. la the Birds, the size of 

 the eggs has been an obstacle for this kind of ob- 

 servation. It has been noticed in the class of Rep- 

 tiles, and in that of Fishes. I have already men* 

 tioned the difficulty which observations encounter 

 in the class of Crustacea and Insects ; in regard to 

 which the data upon the dividing of the yolk are 

 deficient, although it has been observed in the in- 

 ferior Crustacea. It is easily traced in the Worms 

 and Mollusca; indeed it is nowhere easier to ob- 

 serve it, than in these two classes of animals. The 

 phenomenon of dividing of the yolk does not fol- 

 low the same course in every class at the same 

 stages of dev elopment. Perhaps it begins, in some 

 cases, even before the laying of the eggs. This 

 would explain, at least, why it has sometimes not 

 been observed. The process is sometimes slow, 

 sometimes very rapid ; and in this latter case it may 

 easily escape the attention of the observator. Nor 

 must we lose sight of the fact that embryogenie 

 science is a comparatively recent one, and in 

 this department there remains yet much to be 

 done above all, with reference to the study of tis- 

 sues. This should especially be acknowledged, if 

 we consider that it is as late as the year 1834, when 

 Schwann made the discovery of the uniform cel- 

 lular structure of organic tissues, in the animal as 

 well as the vegetable kingdom. 



There are animals, {and it has been more par- 

 ticularly observed among Worms, among Intestinal 

 Worms especially, by Dr. Bagge,) in which the 

 yolk first divides into two halves, which subdivide 

 and subdivide regularly till the whole mass of the 

 yolk is reduced into minute uniform yolklets. The 

 process of this division is also seen in Mollusca, 

 especially among naked Mollusca ; the whole mass 

 dividing into two halves, forming two distinct 

 masses. Next, each will be subdivided into two 



