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. 



Neural tube 



Ectoderm 



^Segmental zxtne 



Splanchnopleure „ , 



entoderm mtochoraal plate 



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- 



Somatic mesoderm 



Primitive node 



.Ectoderm 



omatopleure 



-Entoderm 



Splanchnopleure ' 



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 



