MAMMALIA. 193 



Embryonic membranes and yolk-sack. 



The early stages in the development of the embryonic mem- 

 branes are nearly the same as in Aves ; but during the later stages 

 in the Placentalia the allantois enters into peculiar relations with the 

 uterine walls, and the two, together with the interposed portion of the 

 subzonal membrane or false ainnion, give rise to a very characteristic 

 Mammalian organ the placenta into the structure of which it will 

 be necessary to enter at some length. The embryonic membranes 

 vary so considerably in the different forms that it will be advantageous 

 to commence with a description of their development in an ideal 

 case. 



We may commence with a blastodermic vesicle, closely invested by 

 the delicate remnant of the zona radiata, at the stage in which the 

 medullary groove is already established. Around the embryonic area 

 a layer of mesoblast would have extended for a certain distance ; so 

 as to give rise to an area vasculosa, in which however the blood- 

 vessels would not have become definitely established. Such a vesicle 

 is represented diagrammatically in fig. 147, I. Somewhat later the 

 embryo begins to be folded off, first in front and then behind (fig. 

 147, 2). These folds result in a constriction separating the embryo 

 and the yolk-sack (ds), or as it is known in Mammalian embryology, 

 the umbilical vesicle. The splitting of the mesoblast into a 

 splanchnic and a somatic layer has taken place, and at the front and 

 hind end of the embryo a fold (ks) of the somatic mesoblast and 

 epiblast begins to rise up and grow over the head and tail of the 

 embryo. These two folds form the commencement of the amnion. 

 The head and tail folds of the amnion are continued round the two 

 sides of the embryo, till they meet and unite into a continuous fold. 

 This fold grows gradually upwards, but before it has completely 

 enveloped the embryo, the blood-vessels of the area vasculosa become 

 fully developed. They are arranged in a manner not very different 

 from that in the chick. 



The following is a brief account of their arrangement in the 



o 



Rabbit : 



The outer boundary of the area, which is continually extending further 

 and further round the umbilical vesicle, is marked by a venous sinus 

 terminalis (fig. 147, st). The area is not, as in the chick, a nearly com- 

 plete circle, hut is in front divided by a deep indentation extending inwards 

 to the level of the heart. In consequence of this indentation the sinus 

 terminalis ends in front in two branches, which bend inwards and fall 

 directly into the main vitelline veins. The blood is brought from the 

 dorsal aortfe by a series of lateral vitelline arteries, and not by a single 

 pair as in the chick. These arteries break up into a more deeply situated 

 arterial network, from which the blood is continued partly into the sinus 

 terminalis, and partly into a superficial venous network. The hinder end 

 of the heart is contimied into two vitelline veins, each of which divides 

 into au anterior and a posterior branch. The anterior branch is a limh 



B. E. II. 13 



