MAMMALIA. 



307 



Chick (p. 259). A side view of the blastocyst on the seventh 

 day is shown in Fig. 93.* 



2. General Growth. This is essentially the same as in the 

 case of the Chick (p. 260). The embryo is formed in the 



Fig. 93. Fig. 94. 



Fig. 93. BLASTOCYST OF RABBIT SEVEN DAYS AFTER FERTILIZATION. Side 

 view. Magnified about 10 diameters (after Kolliker). ag, Embryonic 

 area; fje, lower limit of endoderm; below this line the vesicle consists 

 only of a single layer of ectoderm. 



Fig. 94. HEAD OF TEN-DAY RABBIT (after Kolliker}. a, Eye; at, r, 

 and b, atrium, ventricle, and truncus arteriosus of heart; v (upper one) 

 and s, fore- and mid-brains; k', k", k'", mandibular, hyoid, and first 

 branchial arches; o, superior maxillary process of k'; m, mouth. 



embryonic area (area pellucida), and is at first flattened out on 

 the surface of the blastodermic vesicle, just as the Chick is 

 flattened out on the yolk. But the vesicle only contains fluid, 



* The above account of the origin of the germinal layers is the one usually 

 given, but comparison with other forms renders it probable that the wall 

 of the blastocyst outside the embryonic area consists at first of endoderm 

 only, which becomes continuous with the endoderm of that area. If so, 

 the cavity of the blastocyst = an extended archenteron. The lowest mam- 

 mals are developed outside the body of the mother, from large eggs resem- 

 bling those of birds, and there can be no doubt that the rabbit is descended 

 from similar forms, the transition to an intra-uterine mode of development, 

 resulting in loss of the food-yolk. If in a fowl's egg ready for incubation 

 (cf. Fig. 73), we suppose the shell and white removed, and the food-yolk 

 replaced by a much smaller amount of albuminous fluid if, further, we 

 suppose that the lower layer of cells extends right round, and encloses this 

 mass of fluid, then we shall get something resembling the rabbit's blastocyst 

 on the view given in this note. 



