VII 



GENERAL ACCOUNT OF THE DEVELOPMENT OF THE EMBRYO WITH 

 ESPECIAL REFERENCE TO THE EXTERNAL FORM 



i. Changes in the Form of the Egg up to Stage VIII 



The developmental changes undergone by the egg, from the 

 beginning of the cleavage to the close of the formation of the 

 germ layers, have already been described in detail, nevertheless 

 there is one phase of the development which so far has been left 

 out of account, that is, the changes in the external form of the 

 egg. As far as Stage VIII the original outlines of the egg are in 

 a large measure preserved; these have been broken only by the 

 amniotic folds, and in a slighter degree, by the lateral folds. On 

 the other hand the proportions of the egg have been somewhat 

 altered ; during the stages described, it has appreciably shortened, 

 as may be seen by comparing figures I and VII. This shortening 

 is very evident in the actual specimens, in which the chorion is 

 intact, since at Stages VII-VIII there is a space of considerable 

 size left vacant between the embryo and the chorion at the two 

 ends of the egg (Fig. 35D). Examination of early stages (I) 

 shows that at the beginning of cleavage the egg contracts slightly 

 in the long axis, as shown by the appearance of a narrow space 

 between the egg and the chorion at the anterior end and fre- 

 quently at both ends. The amount of contraction does not usually 

 exceed about 5 per cent of the original length of the egg, as esti- 

 mated by the length of the chorion (Fig. 35 A). This shortening 

 of the egg in its long axis was seen in the living egg by both 

 Biitschli (1870) and Kowalevski (1871) and mentioned by them 

 as being one of the earliest phenomena of development. No fur- 

 ther shortening of the egg takes place until the time of the forma- 

 tion of the germ layers, but during this latter period (Stages IV- 

 VII) the egg again shortens rapidly until at Stage VIII the egg 

 (embryo) is now from 20 to 25 per cent shorter than the chorion, 

 relatively large spaces, filled with fluid, being left vacant at both 

 ends of the egg, between the embryo and the chorion (Fig. 356 



95 



