BIGELOW: EARLY DEVELOPMENT OF LEPAS. 131 



others render doubtful the early differentiation of a genital cell in 

 Moina ; but Hiicker ('92, '97) has contributed some important cyto- 

 losical evidence favorable to Grobben's conclusions. 



To summarize the comparison of Lepas with the Copepoda and Phyl- 

 lopoda, it has been pointed out that — 



1. In Lepas, in Moina (Grobben), in Cyclops (Urbanowicz), and 

 probably in the parasitic copepod Lernsea (Pedaschenko) mesoblast 

 originates from ectoblastic cells of the blastoderm around the blasto- 

 pore. In Cetochilus (Grobben) and in Cyclops (Hacker) there is a 

 disagreement with Lepas, in that the entoblast cells are said to originate 

 from cells whose origin and position is similar to those which in the 

 above mentioned forms produce mesoblast. 



2. In Lepas, Cyclops (Urbanowicz) and Lernaea a single entoblast 

 cell, in Cetochilus (Grobben) the " central entoblast " cell, at first lies 

 in the blastopore and it, or its derivatives, are overgrown by the 

 blastoderm. 



3. In Lepas, Cyclops (Urbanowicz), Cetochilus (Grobben) and 

 Lernffia (1) (Pedaschenko) some mesoblast originates directly from 

 the entoblast cell which lies in the blastopore, that is to say, the 

 yolk-macromere is mes-entoblastic. In all of these except Cetochilus 

 (Grobben) mesoblast also originates from ectoblastic cells around the 

 blastopore. 



The foregoing comparisons of the gerra-Iayer formation in Lepas and 

 other Entomostraca in which early differentiation takes place, brings 

 out many points of resemblance. But in some cases there are differ- 

 ences apparently irreconcilable. One can scarcely believe that such 

 contradictory statements as have been summarized in the preceding 

 paragraphs are based upon observations all equally reliable. Renewed 

 investigation of the uncertain points is much needed. The numerous 

 resemblances even from the beginning "of development, make it very 

 desirable that the cell-lineage should in these cases be carefully studied 

 so as to give a basis for accurate comparisons. Until such data are 

 accessible it is unsafe to draw conclusions respecting homologies of cells 

 or even of the germ-layers, 



In many Crustacea there is at the blastopore an immigration of 

 many cells into the cleavage cavity. In some of these cases the cavity 

 .is up to that time filled with yolk. The cell-mass thus formed by 

 immigration into the cleavage cavity is mes-entoblastic, and the meso- 

 blast and entoblast are at first indistinguishable, or at any rate inves- 

 tigators have failed to find distinguishing marks. As examples of 



