vi PREGNANCY PARTURITION PUERPERIUM 231 



name allanto-chorion has been given (6r). The close relation is 

 effected chiefly by connective tissue and by the intra-embryonic 

 blood-vessels, which by means of the mesodermic covering of 

 the allantois are connected with those developed in the internal 

 surface of the chorion and the villi. Thus, while the chorion on 

 the one side comes into relation with the maternal parts and on 

 the other keeps at a distance from the embryo, the allantois, in 

 its capacity of intermediary, renews the relations between the 

 embryo and the chorion. Such relations may be more or less 

 extensive, so much so that some observers on this account have 

 divided mammals into macro-, meso- and micro-allantoids. In the 

 region of distribution of the allantois the placenta is formed. 



We have seen that the splanchnic lamina does not take part 

 in the formation of the arnniotic fold ; it gradually envelops the 

 vitelline substance, while from its mesodermic covering (the 

 splanchnopleure) are developed the blood-vessels which are to 

 help in the absorption of the vitellus. When the splanchnic 

 lamina has completely enfolded the vitelline substance, the result 

 is the formation of the vitelline sac or umbilical vesicle, which 

 only occasionally enters into such intimate relations with the 

 chorion as to form an omphalo-chorion. As a rule the umbilical 

 vesicle gradually becomes smaller owing to the continuous absorp- 

 tion of the deutoplasm it contains (IT). 



Whereas in the sauropsida and lower mammals the formation 

 of the embryonic membranes is effected in the way we have 

 already briefly described, in the primates and in man development 

 follows a different course. In this diverse procedure we see not 

 a process of abbreviation to quote a morphological expression 

 which has no definite physiological value but rather, as Ruffini 

 maintains, one by which the tissues which serve to establish 

 the relation between the product of conception and the maternal 

 parts develop more and more rapidly as we rise in the zoological 

 scale. 



In order to understand aright the formation of the membranes 

 in the human species, we must start from the blastodermic vesicle 

 or blastocyst. We have already seen that a cumulus of internal 

 cells, called from its purpose the embryonic bud or germ, is 

 attached to the upper pole of the blastoderm (Fig. 76), and very 

 shortly a series of flattened cells, arranged in a single layer, is 

 distinguishable below the embryonic bud ; this cellular portion 

 is either arranged against the wall of the blastocyst, thus 

 doubling it, or as is the case of the primates and our own 

 species is confined to a small vesicle below the embryonic bud. 

 Meanwhile a cavity filled with fluid appears inside this bud, and 

 the cells, without becoming detached from the wall of the blasto- 

 cyst, are arranged inside this cavity. Thus the embryonic bud 

 is eventually transformed into a vesicle. 



VOL. v Q 2 



