292 HOLMES. [Vol. XVI. 



to the left of D in forms with reversed cleavage, and to the 

 right of D in forms in which the cleavage is not reversed. 

 Before the division of D, the two macromeres, B and D^ were 

 equal in size, and the anterior and posterior sides of the egg 

 could not be distinguished. The position of d^d or M enables 

 one henceforth to easily locate the posterior side of the o^gg. 

 The primary mesomere soon divides in a nearly horizontal direc- 

 tion, though the cleavage is slightly oblique. This division is 

 completed, as in Siphonaria, before the other cells of the fourth 

 quartette arise. The two mesomeres gradually sink further into 

 the Qg% and lose connection with the surface at about the sixty- 

 four-cell stage. 



One cannot compare the formation of the primary mesoblast 

 in the different pulmonates, in which its origin has been traced, 

 without being struck with the very close similarity of the proc- 

 ess in the several forms. The exact cell origin of the primary 

 mesoblast has been determined in Limax, Physa, Lymnaea, Pla- 

 norbis, and Siphonaria. In all these forms the mesomere arises, 

 shortly after the twenty-four-cell stage is reached, by the oblique 

 division of Z^ ; it lies, from the first, partly pushed into the cleav- 

 age cavity ; it is much larger than the entomere D, and of an 

 opaque appearance ; and it soon divides in a nearly horizontal 

 direction and gives rise to the mesoblastic bands. A similar 

 origin of the primary mesoderm is found in several other gas- 

 teropods, but, with the exception of Umbrella, the mesomere is 

 considerably smaller than the entoderm cell D (Bythinia, Crepi- 

 dula, Neritina, Ilyanassa, Fulgur). There are at present many 

 divergent accounts of the origin of the mesoblast in the mol- 

 lusca. These accounts have recently been reviewed by different 

 writers on the subject (Heymons, '93, Tonniges, '96, Schmidt, 

 '95, Meisenheimer, '96), and fully discussed in Korschelt and 

 Heider's Embryology, so that it would be superfluous to 

 devote space to the subject here. In almost every case, how- 

 ever, in which the cleavage has been followed with sufficient 

 care, the primary mesoderm has been found to arise, as in the 

 above forms, from the posterior macromere D. Yet careful 

 study has failed to discover pole cells in Pahtdina vivipera, 

 but the fate of the cell ^d in this form has never been traced. 



