: EAELY DEVELOPMENT OF LEPAS. 115 



j 



sub-division of nucleated segments from the two yolk segments." Ac- 

 cording to Groom these yolk-segments after separation of the mesoblast 

 divide and form endoderm cells. 



In opposition to this it has been shown in the present paper that the 

 mesoblast clearly does not originate directly from the yolk-cells after the 

 closing of the blastopore ; but from certain cells which have been desig- 

 nated in this account as primary and secondary mesoblasts. The origin 

 of all these cells has been definitely traced. Moreover, evidence has 

 been presented to show that the two yolk-eutoblasts do not begin to 

 divide after the thirty-two-cell stage until at least one hundred and 

 twenty cells are present, of which more than a dozen are mesoblastic. 

 Since the entoblast cells do not divide during these stages, they cannot 

 be the direct progenitors of any of the mesoblast cells. All the evi- 

 dence given seems conclusive and opposed to Groom's interpretation. 



The figures of Groom fail to establish his conclusions regarding the 

 origin of mesoblasts from yolk-entoblasts, for in no case are nuclear spin- 

 dles, the only unimpeachable evidence of such origin, shown. His inter- 

 pretation of the origin of mesoblast cells seems to be based upon their 

 position. In numerous preparations I have seen all the conditions 

 which Groom figures, but I have found no evidence opposed to my in- 

 terpretation of the origin of the mesoblast. Groom did not have trans- 

 parent preparations of entire eggs, and his account of the mesoblast is 

 based entirely upon sections. His figures represent isolated sections, 

 when in many cases only complete series of sections would be convinc- 

 ing. His erroneous conclusion, that the mesoblast is cut off in a series 

 of divisions occurring in a pair of yolk-cells (" meso-hypoblast "), may 

 have resulted from certain conditions which I have frequently noted. 

 Sometimes in stained sections the cell-boundaries of the mesoblast cells 

 are invisible, they appearing to be continuous with the yolk. Under such 

 conditions the mitotic spindles of the mesoblast cells might easily be mis- 

 taken for division of the yolk-cells to form new mesoblast cells. I have 

 seen many such cases which exactly simulated some of Groom's figures, 

 but after removal of the cover glass and restaining, the cell-boundaries 

 of the mesoblast cells and the nuclei of the yolk-entoblasts appeared as 

 usual. 



Nussbaum ( J 90) described the mesoblast in Pollicipes as formed by the 

 division of blastoderm cells surrounding the blastopore before it closes. 

 The mesoblast was said to grow inwards and anteriorly over the yolk. 

 The account of the origin of mesoblast given in the present paper makes it 

 probable that Nussbaum's description is in a general way correct. Had 



