SEGMENTATIOX OF THE GERM. 



677 



a layer of organised cells. This new organised structure, the blasto- 

 derm of Pander, is the future seat of embryonic development. 



A segmenting process of this kind is universal throughout animals as 

 a prelude to the commencement of embryonic development ; but it 

 differs greatly in its extent, and somewhat also in its nature, according 

 to the proportional relation of the directly germinal to the nutritive 

 components of the yolk in different classes of animals. Thus in mam- 

 mals, the process of cleavage appears to be complete, or to involve 

 the whole mass of the yolk protoplasm, with which the germ is 

 coextensive, at least in the first steps of the process; while in the bird's 

 egg, which is pre-eminently merohlastic, the segmentation is restricted 

 in the first instance to the disc of the cicatricula, and the great mass 

 of the yolk substance takes no share in the change. 



In intermediate forms of ova, as in amphibia and osseous fishes the 

 segmenting division extends to a greater or less width over the yolk. 



102. 



Fig. ■492.— Ova OF THE ILvBBrr uxcEiiGoiNa Segmentation in tueir descent THRouan 

 THE Fallopian Tube. (From Bischoff.) f 

 A, tlie ovum from tlie middle part of the tube twelve or fifteen hours after impreg- 

 nation, the germinal vesicle has disappeared, the yolk is contracted, and two hyaline 

 globules are seen in the cavity between it and the zona ; rotation of the yolk took place 

 in the direction of the arrows ; B, ovum a little more advanced, the first segmentation has 

 taken place, a clear globule or nucleus is seen in both the yolk spheres : spermatozoa 

 adhere everywhere to the zona ; C, an ovum four hours later than that shown in B, the 

 second segmentation has taken place ; D, ovum from the lower part of the tube in which 

 the third stage of segmentation is completed, and eight yolk spheres are formed, the 

 albuminous covering is increased in thickness : diameter of the whole, j^th of an inch. 



