232 MR GEORGE BROOK ON THE 



blast for about l-6th of its diameter. It is exactly in this part, and in this 

 part only, that the two layers are originally in close union. 



I need not describe in detail the further advance of the primitive hypoblast, 

 and the gradual obliteration of the segmentation cavity as the result. Through- 

 out the process the parablast is very active in its elaboration of food yolk, and 

 is continually supplying the new layer with more cells. Fig. 23 represents a 

 transverse section of the body axis at a considerably later stage. It shows the 

 primitive hypoblast in close union with the parablast, from which it is derived, 

 but almost completely separated from the cells forming the roof of the 

 segmentation cavity. Fig. 24, which is a more highly magnified view of a 

 portion of this section, shows several important points. The intimate con- 

 nection between yolk and parablast cannot fail to be noted, and the physiolo- 

 gical function of the layer — the elaboration of fresh material for the embryo — 

 is well. brought out. The upper portion of the parablast contains a large 

 number of free nuclei, which agree in every particular with the nuclei of the 

 primitive hypoblast cells. Around some the protoplasm is seen to be con- 

 stricted off to form cells. These are seen in all stages, from those completely 

 embedded in unsegmented protoplasm to those adhering to the parablast only 

 at one point. 



Although at some points the epiblast and the primitive hypoblast are in 

 contact owing to the rapid and prolific growth of the latter, it appears probable 

 that the cells situated above the segmentation cavity give rise to the epiblast 

 only. While the primitive hypoblast remains still undifferentiated, the cells of 

 the epiblast collect, especially in the head region, forming a special thickening 

 for the rudiment of the central nervous system. In the axis of the embryo the 

 primitive hypoblast in the region of the head is pushed to each side by this 

 enormous development, so that, in longitudinal section, the primitive hypoblast 

 appears to cease in the posterior portion of the head swelling. Such a section 

 is shown in fig. 25. The nuclei in the parablast are still prominent, and the 

 two primitive layers are separated by a small space in the neck region, but are 

 united towards the caudal extremity. Fig. 26 represents a portion of the 

 section situated near the tail swelling, more highly magnified. The cells in the 

 epiblast are closely packed together, whereas those in the primitive hypoblast 

 are more loosely arranged. The two layers are not, however, distinctly 

 separated, though a difference in the staining of the nuclei indicates the point 

 of union of the two layers. The parablast still persists as a thin, unsegmented 

 layer of protoplasm closely connected with the yolk, in which numerous free 

 nuclei are embedded. 



At a later stage in transverse section (fig. 30), the primitive hypoblast cells 

 in the median line change their appearance considerably. A number of cells, 

 forming a somewhat circular or slightly quadrate cord, lose their distinct 



