NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 8l 



being first differentiated on the surface of the germinal disk. The 

 first endoderm cells in many of the arthropods scatter through the 

 yolk as independent trophocytes (vitellophags) and the definitive 

 enteron may then be formed either by a reassembling of the cells 

 about the yolk, or by regeneration from intact endodermal rudiments. 

 The mesoderm in some of the Crustacea, Chilopoda, and Chelicerata 

 is proliferated forward from a posterior generative zone very much 

 in the manner of the onychophoran mesoderm, and suggestive of 

 the teloblastic origin of the coeloblast in the Annelida. Among the 

 Crustacea there are in fact a few cases in which the mesoderm takes 

 its origin, at least in part, from a single pair of teloblastomeres derived 

 from the endoderm, as in the cirriped Lepas. The mesodermal telo- 



FiG. 35. — Early stages in the development of a cirriped, Lepas. (Simplified 

 from Bigelow, 1902.) 



A, 8-cell stage, with large yolk-filled posterior cell. B, 30-cell stage, endoderm 

 surrounded by mesoderm comprising a posterior cell (Msd) of endodermal 

 origin, and four cells (msd) of ectodermal origin. C, the posterior mesoblast 

 cell divided into mesodermal teloblasts (MsT). D, near close of gastrulation, 

 but with mesoderm cells still exposed. 



Bpr, blastopore ; Ecd, ectoderm ; End, endoderm ; Msd. endodermal meso- 

 derm ; msd, ectodermal mesoderm ; MsT, mesodermal teloblast ; vCl, yolk- 

 filled cell at vegetative pole of morula. 



blasts of Lepas, according to Bigelow (1902), appear in the 32-cell 

 stage on the posterior lip of the blastopore (fig. 35 C, Mst), and are 

 produced from a single mesoblast cell (B, Msd) that results from 

 the division of a primary yolk-filled blastomere (A, z'Cl) at the 

 posterior pole of the morula. Four other mesoblast cells, however, 

 are formed in Lepas from the ectodermal lips of the blastopore 

 (B, C, msd), and eventually the entire mesoblast sinks into the blasto- 

 pore (D). A separate destiny of the mesoblast from the two sources, 

 entoblastic and ectoblastic, has not been distinguished in Lepas, but 

 it is a point of much interest to note that here the mesoblast completely 

 surrounds the open blastopore between the ectoderm and the endo- 

 derm, a part of it being of endodermal and a part of it of ectodermal 

 6 



