74 



THE STUDY OF CHICK EMBRYOS 



but its ventral portion, the notochord, mesoderm and entoderm, blend in a dense mass of tissue 

 which is characteristic of the primitive node. Laterally the segmental zone and the various 

 layers are differentiated. 



Section Passing through the Hind-gut (Fig. 66). In this embryo the caudal 

 evagination to form the hind-gut has just begun. The section shows the small cavity of the 

 hind-gut in the mid-line. Its wall is composed of columnar entodermal cells and it is an out- 

 growth of the entodermal layer. Dorsal to the hind-gut may be seen undifferentiated cells 

 of the primitive streak continuous dorsad with the ectoderm, ventrad with the entoderm of the 

 hind-gut and laterally with the mesoderm. 



Coelom 



Neura.1 tube 



Echdtrm 



Segmental zane 



<5p!anchnofj/eure < 



Enioderm Noiocnoraa.1 filare 



FiG. 65. Transverse section of a fifty-hour chick embryo through the primitive node cranial to the 



hind-gut. X 50. 



In the chick embryos which we have studied there are large areas developed 

 which are extra-embryonic, that is, lie outside the embryo. The splanchnopleure 

 of the area vasculosa, for instance, forms the wall of the yolk-sac, incomplete in 

 the early stages. The amnion, chorion and allantois are extra-embryonic mem- 

 branes which make their appearance at the fifty-hour stage. These structures 

 are important in mammalian and human embryos and a description of their 

 further development in the chick, where their structure and mode of develop- 



Somafic mezoderm 



node 



'ctocferm 



Coel 



matopleure 



Entoderm 



Splanchnopleure f 



Hind -gut 

 FIG. 66. Transverse section passing through the hind-gut of a fifty-hour chick embryo. X 50. 



ment is primitive, will lead up to the study of mammalian embryos in which the 

 amnion and chorion are precociously developed. 



Amnion and Chorion. These two membranes are developed in all Amniote 

 Vertebrates (Reptiles, Birds and Mammals). They are derived from the extra- 

 embryonic somatopleure. The amnion is purely a protective structure, but the 

 chorion of mammals has a trophic function, as through it the embryo derives its 

 nourishment from the uterine wall. Fig. 67 A shows the amnion and chorion 



