MUSEUM OF COMPARATIVE ZOOLOGY. 207 



The somatopleure of this region, then, is a layer at least two cells in 

 thickness. The splanchnopleure, on the other hand, in this as in later 

 stages, consists of a layer one cell in depth, extending from the ventral 

 surface of the animal to the protovertebral plate. 1 Naturally no sharp 

 line of division can be drawn at this stage between the protovertebral 

 plates and the adjacent portions of the lateral plates. In the section 

 under consideration, the protovertebral plate is rather compact, and it is 

 difficult to indicate with certainty the boundary between the somatic 

 and splanchnic layers. A study of this portion of the mesoderm, how- 

 ever, has convinced me that the coelom (coel. 1 ) is already outlined, and 

 lies in such a position as to leave only a single layer of cells dorsal 

 to it, — a condition which is perfectly evident in later stages. It is 

 indicated by such a distribution of pigment as is seen to the right in 

 Figure 3. 



On following the series of sections farther towards the head, a con- 

 striction of the mesoderm appears beneath the lateral margin of the 

 medullary plate, and the open ccelom is continued into the protover- 

 tebral plate. In a section 1.2 mm. from the posterior end the somatic 

 and splanchnic layers are each but one cell thick in the region of the 

 protovertebral plate. The cells of the somatic layer, which in the proto- 

 vertebral portion are of a high columnar form, become tile-like beneath 

 the pronounced lateral thickening (compare Fig. 1, eras, gn.) of the 

 medullary plate. The somatopleure immediately lateral to the medul- 

 lary plate is rather thick, and becomes thinner both towards the median 

 dorsal and median ventral lines. The regularity of the bounding walls 

 of the body cavity in this region, and the occurrence of a space where no 

 other signs of distortion are apparent, lead me to believe that the separa- 

 tion of the two layers of mesoderm is here perfectly normal, and not, as 

 in more posterior regions, an artificial separation of two closely applied 

 lamella?. 



It is, in general, very difficult to observe karyokinetic conditions in 

 mesodermal or yolk cells, owing to the presence of the large and nu- 

 merous yolk spherules •, but I am reasonably certain that I have ob- 

 served cells in the somatopleural thickening, dividing in a plane parallel 

 to the surface of the layer -, i. e. the cells were dividing in such wise as 

 to increase the thickness of the layer. 



In a section 1.32 mm. in front of the posterior end, the lateral portion 



1 The differentiation of the protovertebrse has not yet begun in this region, 

 and I shall designate the thick masses of mesoderm on each side of the chorda as 

 protovertebral plates. 



