1904.] 



NATURAL SCIENCES OF PHILADELPHIA. 



701 



As compared with the Polychaeta the origin of the mesoblast cell 4D 

 in Dinophihis occurs very early. In fact the segregation of the three 

 germ layers is completed at almost the same instant, since while 3D is 

 dividing to form 4d, 2B is also dividing to form 3b, If, as Mead (1897) 

 urges, the mesoblast belongs to the ideal 64-ceU stage, then 4d should 

 typically arise only after the 32-cell stage. As a matter of fact, it 



A • B 



Fig. IV, A, B, C, D, E, diagrams of embryos of Dinophilus at different stages, 

 illustrating the migration of the ectoderm during the closure of the blasto- 

 pore. The ova are dra-mi as seen from the right side. The approximate 

 limit of the first quartette is shown by a dotted line. The entoderm uncov- 

 ered by ectoderm is stippled. From camera sketches. 



does so arise in all the Polychaeta whose early development has been 

 investigated, and also in Thalassema (Torrey, 1902, 1903). On the 

 other hand, in Capitella 4d belongs to the 29-cell stage, while among 

 the Mollusc a are several forms in which 4d is segregated before the 

 32-cell stage, e.g., Neritina (Blochmann, 1882), Crepidula (Conklin, 

 1897), Physa (Wierzejski, 1897). At the other end of the series 



