230 PHYSIOLOGY CHAP. 



His), well marked in the anterior, lateral, and posterior parts, 

 appear in the embryonic region. When these furrows increase in 

 size and join each other, the embryo appears considerably 

 raised on the blastoderm and surrounded by a single, continuous, 

 and ellipsoidal furrow (Fig. 93, A, B, (7). Beyond this limiting 

 furrow the blastoderm forms a fold (amniotic fold) ; the layers 

 taking part in the formation of this fold are the ectoderm and the 

 somatopleure, which together are designated the somatic lamina, as 

 distinguished from the splanchnic lamina, formed by the union of 

 the splanchnopleure and the endoderm. The splanchnic lamina 

 does not take part in the raising of the amniotic fold, but remains 

 adherent to the vitelline mass which is gradually enfolded so as 

 to form the vitelline sac or umbilical vesicle. Between the two 

 laminae the somatic and the splanchnic is the cavity of the 

 coelom or pleuro-peritoneal cavity, which becomes gradually larger 

 at the point where the somatic lamina rises into the amniotic fold. 

 The zone corresponding to the limiting sulcus, which is called the 

 umbilical region, marks the boundary between the intra-embryonic 

 portion (internal coelom) and the extra- or para-embryonic portion 

 (external coelom) of the coelomatic cavity, from which the 

 allantois grows later on. 



The amniotic folds expand towards the back of the embryo, 

 where, shortly before meeting, they form the so-called amniotic 

 orifice, which, however, soon disappears, because the folds, when 

 they meet, become fused and the amniotic cavity is completely 

 closed. The fusion of the amniotic folds brings about the 

 simultaneous formation of two embryonic membranes, which are 

 distinct and detached : the chorion or serous capsule or membrane 

 of von Baer, and the amnion (F). 



The serous membrane or chorion is covered externally by the 

 ectoderm and internally by the mesoderm. On the external 

 surface of the chorion the chorionic villi are soon formed, as we 

 have already seen. 



In distinction to the chorion, the amnion is covered internally 

 by the ectoderm, and externally by the mesoderm, in which blood- 

 vessels and muscular fibres very soon develop. The cavity of the 

 amnion fills with amniotic fluid, in which the embryo floats and is 

 rocked by the rhythmic contractions of the muscular fibres of the 

 walls of the amnion itself. 



Whilst the amnion and the chorion are undergoing the 

 changes we have described, a hollow diverticulum, covered with 

 mesoderm, is formed on the ventral wall of the caudal portion of 

 the intestines, and forces its way into the external coelom in the 

 form of a vesicle and grows extremely rapidly : this vesicle, which 

 is called the allantois, very soon reaches the chorion ; the two 

 mesodermic surfaces approach one another and become closely 

 adherent, forming, as it were, a single membrane, to which the 



