74 CONK LIN. [Vol. XIII. 



Die erwahnte Zellgruppe liegt in der Medianlinie am Hinter- 

 ende der Schalendruseneinstulpung." In short, this group 

 of cells lying between the teloblasts in Umbrella, corresponds 

 in origin to the most anterior derivatives of 4d in Crepidula, 

 which ultimately lie at the anterior ends of the mesoblastic 

 bands ; the cells which lie between the teloblasts in Crepidula 

 are the most posterior derivatives of 4d. 



Von Wistinghausen ('91), Wilson ('92), and Lillie have 

 observed that a number of small cells are budded off on the 

 surface of the primary mesoblasts in Nereis and Unio. Wilson 

 says that these small cells later wander into the cleavage cavity 

 and form " secondary mesoblast," and Lillie believes that the 

 same thing happens in Unio. It is to be observed that the 

 enteroblasts of Crepidula are, for a long time, uncovered by 

 the ectoblast cells, and that they apparently lie in the layer of 

 ectoblast, and in this regard resemble the small cells described 

 by the authors just mentioned. It is scarcely possible that 

 those small cells are homologous with the enteroblasts in 

 Crepidula, but it is sufficiently obvious that in many cases 

 the history of the so-called "primary mesoblast" has not been 

 followed far enough to determine whether it gives rise to any- 

 thing else besides mesoblast. If a cell arises in the proper 

 place on the posterior side of an egg, and gives rise to a row 

 or band of cells, it is generally supposed to be sufficient ground 

 for calling it the primary mesoblast. I believe that the so- 

 called " primary mesoblast " of many other gasteropods would 

 be found to contain both entoblast and mesoblast if its later 

 history were carefully followed.^ 



In Planorbis and Umbrella the cell 4d arises at the 24-cell 

 stage, as it does in Crepidula; in Unio it appears when 32 cells 

 are present; in Neritina at the 36-cell stage; in Nereis at the 

 38-cell stage; while in Limax it is not separated until the 

 64-cell stage. This apparent difference in the time of its for- 

 mation is due chiefly to the fact that in some cases the ecto- 



iBlochmann's Figs. 62 and 63 for Neritina show two mesoblast bands of two 

 cells each, and between them anteriorly a number of small entoblast cells, some of 

 them closely connected with the mesoblast. These entoblast cells are of doubt- 

 ful origin, and it may be that they correspond to the enteroblast cells of Crepidula. 



