132 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



such conditions may be cited Daphuia, according to Lebedinsky ('9l) ; 

 !Moiua and Daphnia, according to Samassa ('93) ; and many higher 

 Crustacea. 



Such au origin of mesobhist and entoblast is not necessarily opposed 

 to the account which I have given of the gerra-layer formation of Lepas, 

 for differentiation, though not observable, may yet occur in the cases 

 mentioned. Were there not in Lepas peculiarities by which the cells 

 can be distinguished at au early stage, the inmiigratiug mass of cells, 

 composed of entoblast, and of primary and secondary mesoblast, would 

 be correctly described as mes-entoblast, out of which the two layers 

 become later visibly differentiated. If the entoblast cells of Lepas were 

 completely separated from the yolk-mass, as is the case in many other 

 Crustacea, it would perhaps be impossible, in the absence of the easily 

 recognized yolk-laden entoblast, to trace the lineage of the mesoblast 

 independently of the entoblast, and in such conditions it would be nec- 

 essary to consider the immigrating mass of cells as mes-entoblastic. It 

 is probable that some such conditions obtain in some of the Crustacea 

 in which a mes-entoblastic immigration is said to occur. At any rate, 

 germ-layer formation in such cases agrees in essentials with that 

 observed in Lepas. Grobben's ('79) study of Moina suggests that in 

 this genus, at least, the immigrating mass of mes-entoblast may be not 

 entirely undifferentiated as Samassa ('93) supposed. 



There is some evidence that the comparison between Lepas and cer- 

 tain higher Crustacea may be carried still farther than the suggestions 

 offered in the preceding paragraph. In Astacus, according to Reichen- 

 bach ('86), the mesoblast originates at the anterior margin of the 

 blastopore, where the ectoblast joins the entoblast. Keichenbach dis- 

 tinguished in the invagination both yolk-absorbing cells (vitellophags), 

 which enter into the yolk-pyramids, and also the cells forming the 

 entoderm plate. All these cells are said to enter into the mesenteron 

 and liver lobes, and hence the invagination is entoblastic. However, 

 McMurrich ('95, pp. 135, 136) reviews the evidence and suggests that 

 the yolk-pyraraids give rise to some mesoblast. If this proves true, the 

 invagination is to be regarded as mes-entoblastic ; but, in addition to 

 mesoblast so formed from entoblast, other mesoblast cells certainly 

 originate from the blastoderm in front of -the invagination. It follows 

 that there are, as regards origin, two kinds of mesoblast — ectoblastic 

 and entoblastic. 



In other accounts of development of the higher Crustacea there are 

 suggestions of such a double origin of mesoblast, but there is as yet 



