BY JAS. P. HILL AND C. J. MARTIN. 61 



The dark area in the photo-micrograph just external to the 

 somites is the optical expression of this thick somatic layer of 

 mesoderm. The outer limit of the dark area marks the place 

 where the latter becomes reduced to a single layer. 



The ventral ccelom is, in the region of the 1st somite and just 

 anterior to it, coextensive with the thickened portion of the 

 somatic mesoderm seen in surface view, while posteriorly it 

 extends out beyond the j^oint where the latter becomes thin. 

 Further back still the ccelom gradually becomes reduced in extent 

 until in the region of the 9th to the 13th somites the mesoderm 

 is no longer split (fig. 9, mes.). 



Opposite the 14th or 15th somites the mesoderm again becomes 

 split, the ccelom extending close up to the intermediate cell mass 

 (fig. 10). 



Behind the somites the protovertebral zones of mesoderm are 

 directly continuous with the lateral plates, while the splitting of 

 the mesoderm does not occur until some distance out (fig. 12). 



The mesoderm continues some distance beyond the hinder end 

 of the primitive streak, and here the ccelom ic cavities gradually 

 extend inwards towards the mesial line and fuse with each other, 

 so that the ccelom forms a continuous space. In this region the 

 tail fold of the amnion will probably be develo^^ed. 



Primitive streak : In surface view the notochord is seen to 

 become gradually thicker at its posterior end and to terminate 

 finally in a distinct longitudinal thickening situated about the 

 middle of the sinus rhomboidalis. The continuation forwards of 

 this enlargement to join the notochord is the head process of the 

 primitive streak, while behind it is the primitive streak itself, 

 just visible in the photo-micrograph as a whitish line. 



Sections through the primitive streak show that mesodermal 

 cells are being rapidly proliferated off from the ectoderm forming 

 the floor of the primitive groove along its whole extent, and that 

 the lateral wings of mesoderm are directly continuous with this 

 axial streak of cells (figs.. 12, 13, and 24). At the anterior end 

 of the primitive groove ectoderm, mesoderm and entoderm are 

 fused together in the axial line (tigs. 23 and 24) and form the 



