xiv ELEMENTS OF VERTEBRATE EMBRYOLOGY 469 



evidence we should be justified from this fact alone in concluding that 

 mammals have evolved out of ancestral forms in which ;, tiles 



the egg was large and provided with abundant yolk. 



The portion of somatopleure a-ainst whirl, the allantois flattens itself 

 is that which bears the ectoplacenta. The latter, composed throughout 

 the greater part of its thickness of syncytial cytoplasm >. uning 

 scattered nuclei, eats its way into the epithelium linn 

 *97> E) so that it comes into immediate relation with tin under! 

 connective tissue. This is richly supplied with blood, there bcin: 

 place of capillaries, lar u e irn-ulur sinuses full of maternal I.U.d < 

 T 97> V). The protoplasm of the ectoplacenta (e), as it l>urn>w> into the 



i: 







FIG. 197. 



Section through part of blastocyst of Rabbit (8$ days) with the adjoining part of the uterine 

 wall. (Simplified from Duval.) coel, Coelome of embryo ; E, uterine epithelium ; e, ectopl.i. 

 end, endoderm of embryo ; m, myotome ; n.g, neural groove ; s.m, somatic mesoderm ; V, matrin.il 

 blood-vessels of uterine wall. 



substance of the uterine wall, tends to follow especially along the course 

 of these maternal vessels, ensheathing them and destroying the original 

 walls of the vessels so that the latter are now replaced by new walls 

 formed of ectoplacenta (Fig. 198, A and B, V). While the ectoplacenta is 

 invading the uterine wall in the way described, it has itself been invaded 

 on its embryonic face by upgrowths from the underlying mesoderm 

 formed by the fusion of the somatic mesoderm with the splanchnic 

 mesoderm covering the allantois (Fig. 198, B). This intrusive mesoderm 

 is richly supplied with blood from the allantoic vessels. 



The placenta now takes on a distinctly columnar appearance when 

 observed in microscopic sections, the wide tubular maternal blood 

 sinuses (Fig. 198, C, V) with their wall of ectoplacental protoplasm 

 alternating with masses of embryonic connective tissue (s.m) containing 



