66 



THE STUDY OF CHICK EMBRYOS 



the aortas smaller, and a few sections caudad they disappear. Laterally the somatopleure 

 and splanchnopleure are straight and separated by the slit-like coelom. 



Section through the Notochordal Plate, Cranial to the Hind-gut (Fig. 68). With the 

 exception of the ectoderm, the structures near the median plane are merged into an undif- 

 ferentiated mass of dense tissue, the notochordal plate. The cavity of the neural tube and 

 its dorsal outline may, however, still be seen. Laterally the segmental zone and the 

 various layers are differentiated. 



Neural lube 



Coelom 



Ectoderm 



Segmental zone 



Splanchnopleure / Notochordal plate 



Entoderm 



FIG. 68. Transverse section of a fifty-hour chick embryo through the notochordal plate, 



cranial to the hind-gut. X 50. 



Section through the Hind-gut and Primitive Streak (Pig. 69). In this embryo the cau- 

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

 of the hind-gut in the midplane. Its wall is composed of columnar entodermal cells and 

 it is an outgrowth of the entodermal layer. A few sections cephalad in the series, the hind- 

 gut opens by its own intestinal portal. 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. 



Somatic mesoderm 

 Codom 



Primitive streak 



Ectoderm 



Somatopleure 



Splanchnopleure' / \""~ 



Hind-gut Entoderm 



FIG. 69. Transverse section through the hind-gut of a fifty-hour chick embryo. X 50. 



Extra-embryonic Structures. 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 membranes 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 devel- 

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

 which the amnion and chorion are precociously developed. 



