EARLY STAGES OF THE DEVELOPMENT OF THE FOWL. 931 



Having proceeded thus far with the development of the chick, we may 

 DOW return to the early stages of the development of the Mammalian ovum. 

 The ovum of the Mammal differs from that of the bird, in not presenting 

 the same differentiation into white and yellow yolk and cicatricula, but 

 resembles the cicatricula in undergoing complete segmentation. It is origi- 

 nally composed of yolk spheres diffused in protoplasm, and contains a ger- 

 minal vesicle and spot. As the egg ripens the vesicle comes to be nearer and 

 nearer to the surface, and before impregnation occurs, and probably as a 

 consequence of the contractile power of the protoplasm, is squeezed out from 

 it, and lies between it and the zona pellucida. Here it undergoes segmenta- 

 tion, but its further changes are unknown, except in so far that the products 

 of its division do not appear to stand in any genetical relation to the nuclei 

 of the primary segmentation spheres. 1 These are derived from the division of 

 the yolk, which takes place in a manner essentially similar to that of the 

 cicatricula of the Avian egg (Figs. 335 and 336), the ovum ultimately 



FIG. 336. 



Later stage in the Segmentation of the Yolk of the Mammalian Ovum : at A is shown the " mulberry- 

 mass" formed by the minute subdivision of the vitelline spheres ; at B, a further increase has brought 

 its surface into contact with the vitelline membrane, against which the spherules are flattened. 



acquiring a mulberry-like aspect, and by the time of its arrival into the 

 uterus the whole cavity of the vitelline membrane or Zona pellucida being 

 occupied by minute spherules of yolk. 2 The outermost cell-like products of 

 the segmentation of the ovum assume a definite arrangement, and form a 

 kind of membrane within or beneath the Zona pellucida, named by Bischoff 

 the blastodermic, vesicle. This increases rapidly in size, and becomes filled 

 with fluid furnished by the uterus. When it has attained to I", in the ovum 

 of the Rabbit, a round spot begins to be distinguished by its white opaque 

 appearance; this is the Area germinativa (Plate I, Fig. 6). The Area ger- 

 minativa continually increases in extent and thickness by the formation oi 

 new cells, and soon exhibits a differentiation into an upper and a lower layer 

 (Plate I, Figs. 5 and 6), and the lower layer again speedily divides into two. 

 Thus, the same succession of layers an outer or epiblastic, a middle or me- 



1 Oellacher, Max Schultze's Archiv, Bd. viii, p. 1, 1872. 



2 A rotation of the yolk at this period, for which the presence of oxygen appears 

 to be a necessary condition (see Dr. Kansom, Humphry and Turner's Journal of 

 Anatomy, vol. i,"l867, p. 237) was observed by Dr. Ransom in the ova of the Gaster- 

 osteus, and was soon after noticed in the Pike by Reicbert, in the frog by Ecker, 

 and subsequently in the Rabbit and Guinea-pig by Bischoff. The cause of this sin- 

 gular movement is still unknown, and though Bischoff stated that he had seen cilia in 

 the rabbit on the outer surface of the yolk, he was unable to satisfy himself of their 

 presence in the guinea-pig. 



