212 



WILSON. 



[Vol. \\\ 



ventral plate consists of twelve principal cells, arranged in two 

 longitudinal rows of three each on each side of the median line. 

 Anteriorly are the two primary mesoblasts with their first-formed 

 derivatives (F-F) ; posteriorly are four teloblasts {X-X), and 

 four smaller cells derived from them, as shown in the figure. 



In the next stage the four smaller Jf 's divide obliquely, while 

 four smaller cells are produced anteriorly from the four larger 

 .Y's. The four F's are now rapidly pushed into the interior, 

 being overgrown by the progeny of X, which advance upon them 

 from behind (Fig. 6), and by the micromeres advancing from the 



X" 



Fig. 6. — View of a still later stage in optical luiigitudinal section from the right- 

 hand side. I.-I., the first cleavage-plane; Ec, the ectoblast derived from the micro- 

 meres; Pr, the prototroch; OD, the oil-globules; Y, primary mesoblast in process 

 of pushing in; X-X, progeny of the first pro-teloblast. The blastopore, which was 

 situated just anterior to Y, has been closed in by the micromeres. 



sides and in front. As they are pushed in, the smaller F's 

 divide, and a second pair of smaller cells are separated from the 

 two large F's (primary mesoblasts), which are now scarcely larger 

 than the cells to which they have given rise, or than the super- 

 ficial cells (descendants of X) among which they are imbedded. 

 After the next division the teloblasts become indistinguishable, 

 and the mesoblast-bands seem to fuse with the ventral plate ; 

 so that an examination of the embryo at this period, without 

 a knowledge of its earlier history, would certainly lead to the 

 conclusion that the mesoblast-bands arise by proliferation from 



