No. 3.] PODARKE OBSCURA VERRILL. 433 



Among the mollusks Conklin (No. 5, a) has shown that 4d 

 in Crepidula is more than half entoderm. It is not impossible 

 that a portion of this cell in Unio may have a similar fate, and 

 a close relation between mesoderm and entoderm has been 

 described for Cyclas (No. 28) and Patella (No. 26), though the 

 precise cell origin of the mesentoblasts in the latter cases was 

 not determined. These observations seem to indicate a lack 

 of uniformity in the mode of origin of the mesoblast and also 

 show an apparently close relation between mesoblast and ento- 

 blast. To this point I shall return later. (See p. 449.) 



The other members of the fourth quartette apparently invagi- 

 nate. (See their small superficial area in PI. XXXVIII, Figs. 

 33 and 35.) In PI. XXXVIII, Fig. 34, the nucleus of 4a shows 

 underneath the 2a cells. In some cases I was unable, in the 

 stained specimens, to see the outlines of the cells. This fact 

 led me to the erroneous statement in my preliminary paper 

 (No. 29, a) that they invaginate before dividing. As a mat- 

 ter of fact, the cells come to the surface and divide equally 

 (PI. XXXVIII, Fig. 36), and then immediately invaginate, 

 entering into the wall of the archenteron. 



TJie Fifth Quartette. — A fifth series of four cells arising 

 from the entomeres has been called the fifth quartette of micro- 

 meres by other workers, and in my preliminary note (No. 29, a) 

 I emphasized the fact that in Lepidonotus, as in Podarke, such 

 a quartette appears. The term, however, is misleading, since 

 the cells thus formed are entodermal in destiny ; and we must 

 regard this division as corresponding to the division which, in 

 other forms, takes place after invagination. The only differ- 

 ence between the two cleavages is that one takes place at the 

 surface and the other below it. The one is as truly a fifth 

 quartette of micromeres as the other. 



This division in Podarke is shown in PI. XXXVIII, Fig. 33, 

 occurring in C and D quadrants, and later it appears in quad- 

 rants A and B (PI. XXXVIII, Fig. 34). The eight cells thus 

 formed, together with six derived from the division of three 

 members of the fourth quartette, make up an invaginating plate 

 of fourteen cells, which rapidly invaginates, its cells meanwhile 

 dividing again ; and the alimentary canal of the embryo is 



