130 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



entoderm " are said to be the primitive raesoblast cells, and the two 

 posterior products ectodermal. 



It appears that the " central entoderm " cell of Grobben is probably 

 the single entoblast cell to which Urbanowicz refers. The blastoderm 

 cells lying laterally and anterior to the entoderm cell in Cyclops are 

 said by Urbanowicz to give rise to mesenchyme, while Grobben in 

 Cetochilus and Hacker in Cyclops find entoderm originating from cells 

 in corresponding positions. It is probable that this contradiction arose 

 from failure to follow the germ-layers into the ultimate organs. The 

 figures of Cetochilus by Grobben and those of Cyclops by Hacker do 

 not give conclusive proof regarding the fate of the cells which they con- 

 sider endoderm. I have not seen the original figures by Urbanowicz. 

 The differences between these authors will probably be adjusted when 

 the later history of the mesoblast and entoblast is more accurately 

 traced. 



The cell posterior to the " central endoderm " cell in the thirty-two- 

 cell stage of Cetochilus is said by Grobben to form the mesoblast and 

 also to contain some ectoblast. This latter point must still be regarded 

 as problematical, for Grobben's figures do not give convincing proof. 

 It is possible that the cell in question may be wholly mesoblastic, in- 

 stead of only partly so. However, the important point is that this cell 

 appears to originate in connection with the " central endoderm " cell. 

 Accordingly mesoblast in Cetochilus originates from etitoblast ; a con- 

 dition certainly existing in the case of the barnacle Lepas, and the 

 studies of Urbanowicz make it appear probable that such is also the 

 case in Cyclops. 



Grobben's ('79) account of the development of the phyllopod Moina 

 agrees with Urbanowicz's account of Cyclops and my own account of 

 Lepas as to the formation of ectoblastic mesoblast from blastoderm cells 

 bounding the blastopore laterally and anteriorly. But in a position 

 corresponding to that of the entoblast cell of Lepas and Cyclops there 

 is in Moina a " primitive genital cell," and the entoblast is said to be 

 developed from a cell lying immediately posterior to it. It should be 

 mentioned here that Samassa ('93), while agreeing essentially with 

 Grobben's description of cleavage stages, failed to find evidence of such 

 early differentiation. With respect to this result it must be considered 

 improbable that the visible peculiarities of the cells in the region of the 

 blastopore in cleavage stages are without significance. It seems more 

 probable that the peculiar features of certain cells do represent early 

 differentiations, as Grobben claimed. The results of Samassa and 



