22 WILSOX. 



one noteworthy respect, namely, that the division of the pos- 

 terior " macromere " is variable, only rarely dividing equally 

 (Fig. 6, F^ and as a rule dividing unequally, giving rise to a 

 smaller cell (4<^^ Fig. 6, E^ that is typically formed obliquely 

 •towards the right as seen from the side (/. c, in a leiotropic or 

 anti-clockwise spiral.^ From this it appears that the form of 

 cleavage in the fourth quartet of Discoavlis, which agrees so 

 exactly with that of the annelids and mollusks, appears as only 

 an occasional variation in Lcptoplana, though even here the 

 posterior ''macromere" is always the first to divide. 



As regards the fate of these cells, the inequality of ^d^ and 

 A^d- (often veiy marked) is itself indirect evidence that they do 

 not give rise to symmetrical mesoblast-bands as in the higher 

 types and I find no evidence that either of them gives rise to 

 mesoblast-cells. Both seem to have the same fate as the other 

 entoblast-cells, with which they exactly agree in deutoplasmic 

 structure, and enter into the formation of the archenteron as 

 Lang has shown in the case of Discocoelis. Can we neverthe- 

 less regard them as homologous to, or rather as the prototypes 

 of, the primary mesentoblasts of the annelids and mollusks ? 

 When ^ve reflect on the facts, reviewed in the first part of this 

 paper, we may hesitate to answer this question in the negative. 

 For we have seen reason for the conclusion that the primary 

 mesoblasts of annelids and gasteropods have arisen historically, 

 as they arise ontogenetically, /"r<^;// tJic posterior part of tJic arcJi- 

 entcron ; and we have traced the entoblastic elements of the 

 posterior cell of the fourth quartet from a minute and apparently 

 functionless vestige (Aricid) back to a group of large and im- 

 portant cells (Crepididd). I think we should consider the pos- 

 sibility, if only as a working hypothesis, that in ancestral types 

 the entoblastic elements of the posterior cell of the fourth quartet 



1 Typically — i. e., in probably ninety per cent, of the cases observed, the division 

 is markedly unequal — often much more so than in Fig. 5, ^. In a few cases the 

 direction of division is reversed, the smaller cell, 4^2 being found towards the left 

 (dexiotropic spiral). Sometimes the division is equal and vertical as in Discoccclis ; 

 more rarely it is horizontal and either equal or unequal. I believe all these varia- 

 tions occur in normal embryos. A considerable time after the formation of 4^/^ the 

 other macromeres begin to divide unequally and irregularly, and all the macromeres 

 ultimately break up into smaller rounded cells, heavily laden with deutoplasm. 



