lxxxiv REPORT — 1877. 



When, as before mentioned, the germinal vesicle has been in part ex- 

 truded or lost to sight, the -whole yolk-substance of the ovum forms a nearly- 

 uniform mass of finely granular protoplasm, enclosed within the external 

 cell-membrane. Within a few hours later a clear nucleus has arisen in this 

 mass. To this more definite form of organization assumed by the germinal 

 substance of the future animal, which is about to be the subject of the seg- 

 menting process, the name of the first segment-sphere may be given. 



By the process of cleavage which now begins, this first segment-sphere 

 and its nucleus undergo division into two nucleated spheres of smaller size, 

 the whole substance of the yolk, in a holoblastic ovum, such as that of the 

 mammal, being involved in the segmenting process. 



The second ■ stage of division follows after the lapse of a few hours, and 

 results in the formation of four nucleated segment-spheres ; and the process 

 of division being repeated in a certain definite order, there result in the 

 succeeding stages (that is, the third, fourth, fifth, and up to the tenth) the 

 numbers of 8, 12, 16, 24, 32, 48, 64, and 96 nucleated yolk-spheres, germ- 

 spheres, or formative cells. 



In the rabbit's ovum the tenth stage is reached in less than three days ; 

 and as during that time the size of the whole ovum has undergone very little 

 increase, it follows that the spheres of each succeeding set, as they become 

 more numerous, have diminished greatly in size. These segment-spheres 

 are all destitute of external membrane, but are distinctly nucleated ; and their 

 protoplasmic substance is more or less granular, presenting the usual histo- 

 logical characters of growing cells. 



By the time that segmentation has reached the seventh or eighth stage, 

 when 32 or 48 spheres have been formed, the ovum has assumed the 

 appearance of a mulberry, in which the oi;ter smaller spheres, closely massed 

 together, project slightly and uniformly over the whole surface ; while the 

 interior of the ball is filled with cells of a somewhat larger size and a more 

 opaque granular aspect, also resulting from the process of segmentation. 



Already, however, the mutual compression of the spheres or cells on the 

 surface, by their crowding together, has led to the flattening of their adjacent 

 sides ; and by the time the tenth stage is reached, when the whole number 

 of the cells is about 96, the more advanced superficial cells, having ranged 

 themselves closely together, form a nucleated cellular layer or covering of 

 the yolk, enclosing within them the larger and more opaque cells, derived 

 like the first from the segmenting process. In a more advanced stage, the 

 deeper cells now referred to having taken the form of an internal layer, there 

 results at last the bilaminar blastoderm or embryonic germinal membrane. 



The process of partial segmentation, such as occurs in the bird's egg, 

 though perhaps fundamentally the same as that of the mammal previously 

 described, stands in a different relation to the parts of the whole yolk or 

 egg, and consequently differs in its general phenomena. The segmentation 



