468 



In an embryo ten days old the ventral thickening widens some- 

 what and the large dorsal cells have pushed farther around towards 

 the ventral side. This process continues in succeeding stages, and in 

 a cross-section of an embryo at the age of fourteen days we have 

 the appearance shown in Figure 19. Here the ventral thickening has 

 become wider and somewhat thinner and is composed of two layers : 

 an outer, compact ectodermal layer ; and an inner, somewhat looser 

 layer of mesodermal cells. The large cells from the dorsal side have 

 grown around a little farther toward the ventral side. These cells, 

 together with the small cells which form a network among them, 

 form a layer which is beginning to separate slightly from the thin 

 ectoderm dorsally and laterally, leaving a small space occupied by 

 scattered cells. The invagination of the proctodseum has not yet 

 developed at this stage. 



In a somewhat later stage, represented by Figure 20, (the exact 

 age of these later stages can not be given,) the ventral thickening has 

 widened somewhat, and in the middle of this thickening there are 

 two longitudinal elevations, inside of which we see the beginnings of 

 the nerve cord. These appear in cross-section as two circular areas 

 of cells overgrown with the ectoderm. The layer of mesoderm cells 

 which we found lying just below the ectodermal thickening in the 

 fourteen-day stage has here split into two parts, one part lying on 

 each side of the developing nerve cord. 



The layer of large cells which has been growing around from 

 the dorsal side has now reached the median ventral line, and the two 

 edges unite to form a circular layer. The ultimate fate of these cells 

 is now evident. They, together with the network of small cells which 

 grew out from the posterior end of the germ band, and with the 

 scattered cells of the inner layer of peripheral protoplasm, have 

 formed the mesenteron. These cells which were enclosed in the 

 network and were originally very large, are now much smaller, and 

 there is no longer a clear distinction between the various kinds of 

 cells which went to make up the layer. The mesenteron has now 

 completely separated from the surface ectoderm, leaving a well-de- 

 veloped body cavity. 



The mesenteron ends blindly at both ends. Its shape may be 

 seen from Figures 22 and 24. Its wall is thick, appearing in sections 

 as a protoplasmic network enclosing the cells. This network is es- 

 pecially noticeable on the inner border. 



By this time, appendages have been formed and the invaginations 

 of the stomodseum and proctod?eum are well developed. The de- 

 velopment of the appendages and the further development of the 



