466 Mr. S. Hatta. Mesodermic Origin and the Fate of the 



ectoderm ; whilst the first somite is represented by the anterior blind end* of 

 the epithelial archenteron, which is folded off from the rest as a median 

 unpaired pocket. This somite is so small that it is practically destitute of 

 the lateral plates. 



In the two posterior of the three somites the outer wall, which corresponds 

 to the cutis layer of a postotic somite, gives off free cells which push their 

 way between the ectoderm and the somitic layer of the lateral plates (text- 

 fig. 4). The free cells are quickly increased to some extent by the cells 



Text-fig. 4. — Diagrammatic representation of a transverse section through the second 

 somite, showing the proliferation of the mesectoderm cells from the lateral layer of 

 the somite, me.c, mesectoderm cells (shaded) ; ?ic, nerve cells ; sm.2, second somite 

 standing still in connection with the pharynx ; tr., placode for trigeminal ganglion. 

 For other letters, see the explanation of the previous diagrams. 



budded out from the lateral layer of the somitic fold. The somites are cut 

 off from the archenteric roof only on the fifth day, while the cell-proliferation 

 begins at the early part of the fourth day, therefore earlier than the stage at 

 which the ventro-lateral edge of the postotic scleromyotomes begins to be 

 produced into the mesectoderm. The free cells soon make up a thick 

 columnar epithelium which represents the mesectoderm of this preotic 

 region. In contrast to that in the postotic region, the mesectoderm is here 

 not confined to the lateral part, but is spread into the ventral wall of the body, 

 which gives rise to the stomodseum by invagination. 



At the same time, but on a smaller scale, the cells wander out of the 

 lateral layer of the somites into the space between the medullary canal and 

 the ectoderm opposite and above the ganglion placodes. They do not assume 



* The first somite may not be confounded with the anterior blind sac of the pharynx 

 of Kupffer ; for the explanation of both the structures I refer to my paper above given 

 (146). 



