DEVELOPMENT OF THE MAMMALIAN EMBRYO. 



83 



OF THE DEVELOPMENT OF THE EMBRYO. 

 1. In Mammiferous Animals.* 



FROM the observations of recent embryologists, especially of Professor 

 Bischoff,f it would appear that in the earlier periods of its formation the 

 Mammalian embryo presents a close resemblance to the embryo of the chick, 

 and that (as was shewn by PreVost and Dumas) { the process of development 

 in each takes place according to the same general plan. We have already 

 traced the changes which ensue in the Mammiferous ovum subsequent to 

 impregnation, as far as the formation of the area germinativa, and the 

 separation of the germinal membrane into two layers. At its first appear- 

 ance the area germinativa has a rounded form, but it soon loses this and 

 becomes oval, then pear-shaped, and while this change in form is taking 

 place, there gradually appears in its centre a clear space or area pellucida 



Fig. 10.$ 



(fig. 10, C), bounded externally by a more opaque circle which subse- 

 quently becomes the area vasculosa (B). In the formation of these two 

 spaces, both the serous and mucous laminse of the germinal membrane take 



* Chapter iii. p. 1560 of Muller's Physiology. t Operibus citatis. 



J Muller's Physiology, p. 1566. 



Fig. 1 0. Portion of the germinal membrane of a bitch's ovum, with the area pellucida 

 and rudiments of the embryo ; magnified ten diameters. A. Germinal membrane. B. Area 

 vasculosa. C. Area pellucida. D. Laminae dorsales. E. Primitive groove, bounded 

 laterally by the pale pellucid substance of which the central nervous system is composed. 

 After Bischoff. (Entvvickelungs-geschichte des Hunde-eies.) 



