126 WHITMAN". [Vol. I. 



while the neuroblasts and nephroblasts, lying in front of and 

 differing but little in size from the cells which they produce, are 

 discoverable only by careful microscopical examination. This 

 accounts for the fact that they have hitherto been overlooked, 

 and for the widespread belief that the mesoblasts are the sole 

 proliferators of the germ-bands. The origin of the mesenteron, 

 as described by Kowalevsky, is another evidence in favor of 

 identifying the posterior macromere in Rhynchelmis with that 

 of Clepsine. 



Let us now consider the history of this macromere more in 

 detail, in order to see how far observation supports the com- 

 parison with Clepsine. Vejdovsky has extended and corrected 

 Kowalevsky's account of the cleavage in many particulars, and 

 I shall therefore be guided by his statements. 



The first cleavage-plane divides the egg into unequal parts, 

 the larger of which receives the remnants of the polar discs of 

 protoplasm, as in Clepsine. The second cleavage begins first 

 on the larger segment, but passes to the small segment before 

 the large one is completely divided, so that a three-cell stage 

 does not really exist. All this seems at first sight to be in 

 perfect accord with the first two cleavages in Clepsine. But 

 there is one difference in regard to the second cleavage which 

 threatens to upset the whole comparison. This cleavage does 

 not divide the still visible remnant of the polar disc, but 

 runs to the left of them, while in Clepsine it runs to the right. 

 This difference introduces completely new axial relations, 

 analogous to, but not identical with, those in Clepsine, as 

 will be seen by comparing Diags. 7 and 8. It makes 90^ dif- 

 ference in the direction of the embryonic axis, for b now becomes 

 the posterior macromere, and is destined to play the same r61e 

 as the macromere x in Clepsine. The embryonic axis now bisects 

 a and b (Diag. 8), instead of c and x, as in Clepsine (Diag. 

 7). Thus arises a very serious difficulty in the way of identify- 

 ing the macromeres. I see only two ways of meeting this poixit. 

 I. There is, of course, a possibility of error in observation, 

 and the liability to error is all the greater, as Vejdovsky does 

 not appear to have given any attention to the relations we are 

 now considering. 2. If he is right in placing the second cleav- 

 age-plane to the left of the disc, it will still be possible to identify 

 the macromeres, provided the order of the first two cleavage- 



