KOFOID: DEVELOPMENT OF LIMAX. 97 



matter. It is possible that this may represent the segmentation cavity, 

 though it does not appear to be present in all cases." This is in a stage 

 preceding the formation of the " sixth generation " of micromeres and 

 the appearance of the mesoderm. Brooks ('78) figures at a late stage a 

 cavity in Urosalpinx similar to that found by Bobretsky in a late Btage 

 of Xassa. Patten ('&6) does not figure any segmentation cavity in 

 Patella, but in the later stages, before gastrulation, a medium-sized 

 central cavity is present. Conklin ('91) finds in C'repiduia, "at an early 

 period, a trace of a segmentation cavity, which, however, is soon 

 obliterated." 



II. Fresh ■ Water Forms. 



Blochmann ('81) does not discuss the subject of the cleavage cavity 

 in Neritina, but it is evident from his figures that, if it is present at all 

 in the earlier stages, it does not attain a great size. Xcritina contains 

 a large amount of yolk, and this may have some effect on the cleavage 

 cavity. In the late stages a small cavity appears between the ectoderm 

 and the macromeres. 



No mention is made by Sarasin ('82) of a cleavage cavity in Bithynia 

 until the close of the cleavage period. Sections of the early stages were 

 not cut. Erlanger ('92) finds a large cavity present at the close of seg- 

 mentation, i. e. before the formation of the mesoderm and when the 

 blastomere contains, according to his estimate, at least sixty cells. In 

 Paludina vivipara, Lankester ('76) finds in a late stage of cleavage " a 

 central space or cleavage cavity." A cavity of considerable size is also 

 figured by him as present at the time of gastrulation. On the other 

 hand, neither Biitschli ('77) nor Blochmann ('83) succeeded in finding 

 in this species any cavity in the cleavage stages examined by them, nor 

 more than a mere slit-like cavity between the layers at the time of 

 gastrulation. Erlanger ('91), however, finds a large cavity in the 

 gastrula stage, and it is into this cavity that the mesodermal pockets 

 described by him are evaginated. If Paludina has an ephemeral 

 recurrent cavity similar to that of Limax, the apparently discordant 

 observations of Lankester, Biitschli, and Blochmann would be easily 

 harmonized. 



I have myself watched the cleavage of the eggs of Amnicola limosa, 

 and find that they present a typical recurrent cavity, precisely like that 

 of the fresh-water pulmonates. The eggs of Amnicola are enclosed in 

 capsules and are surrounded by a jelly-like albumen. They contain a 

 small amount of yolk, and cleavage is not so unequal as it is in 

 Neritina. 



VOL. XXVII. — NO. 2. 7 



