132 THE DEVELOPMENT OF 



endoderm, while his endoderm is clearly the splanchno- 

 pleure of the authors of the "Coelomtheorie." The ob- 

 servations of Korotneff' ('85) and Patten ('84) are less. 

 easily explained, for both state that cells arise from all 

 parts of the blastoderm and pass into the yolk to form the 

 endoderm, and that before this "diffuse gastrulation" more 

 are left in the yolk. Were it not for this, the Statements 

 and figures of Patten (p. 572, pl. XXXVIIB, fig. 12) that 

 the primitive streak "gives rise to a part of the endoderm 

 and all of the mesoderm," might be readily understood as 

 the last stage in a process of gastrulation previous to the 

 condition of affairs which we have supposed above. Ko- 

 rotneff's recogiiition of two mesodermal elements — myo- 

 blasts and mesenchyma — and his description of their mode 

 and places of origin still further complicate the matter and 

 niake a reexamination of his results desirable. 



While upon this subject of arthropod segmentation and 

 gastrulation it may be well to refer to auother point which 

 seems to have caused considerable trouble, and for which 

 an explanation is apparently more easy than in the cases 

 already discussed. In afew arthropods — Scorpio (Mets- 

 chnikoff, '71), Nebalia (Metschnikoff, '68), Mysis (Van 

 Beneden, '69^),Oniscus (Bobretzky, '74), and Cymothoa 

 (Ballar, '78), the segmentation is of a meroblastic char- 

 acter, recalling quite strongly that of the meroblastic ver- 

 tebrates or even of the teleosts. While we greatly need 

 new observations upon these forms I think the facts in our 

 possession fully Warrant us in regarding them as not great- 

 ly different from the more normal types. 



In all arthropod eggs there is a certain amount of po- 

 larity and in some it becomes quite marked, the cells 

 appearing at one point more abundantly and earlier than 

 at others. This was noticed by Locy in Agelena, but is 

 more apparent in the cases of Aphis, Gryllotalpa, and, it 



