Il6 MCMURRICH. [Vol. XI. 



they assume their characteristic function. For convenience, I 

 distinguish here these vitellophag cells from the remaining 

 elements of the mes-endoderm ; their significance will be dis- 

 cussed in another section of this part of the paper. 



The mes-endoderm cells do not remain irregularly scattered 

 under the naupliar ectoderm very long, but one soon finds 

 them arranging themselves in two bands which diverge as 

 they are traced forward, and extend from what was originally 

 the blastopore region to beneath the ocular lobes, forming 

 what have been termed the mesoderm bands, and lying 

 beneath the regions of the ectoderm in which the cells of 

 that layer are somewhat concentrated, as shown in Fig. 25 

 for Jaera and in Fig. 38 for Asellus. In addition in Jaera a 

 transverse band extends across from about the anterior end of 

 one mesoderm band to the other, so that occupying the central 

 part of the naupliar region of the embryo there is a triangular 

 area in which there are practically no mesoderm cells (see 

 Fig. 25) and in the anterior portion of which, it may here be 

 stated, the stomodaeal invagination will take place. I have 

 not been able to follow the development of these mesoderm 

 bands in Porcellio or Arinadillidmm, but in these as well as 

 in the other forms studied their existence is clearly indicated 

 in later stages when the naupliar limbs begin to form. 



When these stages are reached, the mesoderm cells are 

 found to have arranged themselves into groups, which, as the 

 limbs grow out, migrate into their interior, and multiplying 

 there form solid mesodermal axes for them. Other mesoder- 

 mal cells are scattered beneath the Anlagen of the nervous 

 system and beneath the region where the stomodaeal invagina- 

 tion is preparing, but no special mesodermal groups were 

 found immediately external to the points of origin of the 

 naupliar limbs. At this stage a differentiation of endoderm 

 cells appears owing to the formation of a special group of cells 

 on either side at about the level of the first maxillae, these 

 cells forming the Anlage of the liver lobes. They early ar- 

 range themselves in the form of a hollow sphere, which is not 

 complete, however, being open towards the yolk, which the 

 cells proceed to digest. I may say here that I have found 



