260 C. O. WHITMAN. 



wedged apart by the dorsal (.r), and the ventral (c) blastomeres 

 above and below, though more below than above. Soon (6 h. 30 

 min., fig. 20) the upper angles of the large dorsal blastoinerc (.?•) 

 lengthens towards the ventral blastomere (c), and at the same 

 time a constriction begins to separate this bud-like extension 

 from the mother-cell (,t), ending in the formation of the first 

 ectoblast. This ectoblast during the next thirty minutes is 

 pushed towards the left by a prolongation from the upper angle 

 of the right blastomere (d), and finally becomes so imbedded in 

 the left blastomere (a) that one would easily mistake it for a 

 prolongation similar to that of the opposite side (fig. 21, 7 h.) 

 had not one followed the process from beginning to end. It is 

 probable that in this way Robin was led into error. I have 

 followed this entire process without interruption many times and 

 always have found the phenomena repeated as given above. I 

 can affirm also that the process is identical in the four species 

 that I have studied. Thirty minutes later (fig. 22, 7 h. 30 min.) 

 the formation of the first two ectoblasts is already completed, 

 and that of the second two is in progress. The latter are formed 

 simultaneously from the lateral blastomere {a) and the ventral 

 blastomere {c). The result is four ectoblasts lying exactly in 

 the boundary lines of the four large quadrants where these lines 

 cross, thus presenting a cruciform arrangement. 



(c) Formation of the two Mesoblasts and the primary Neuro- 

 blast. — The result of the cleavage thus far is represented by 

 four large primary cleavage-spheres (a, h, c, x) and four small 

 ectoblasts, one of which was ])roduced by each blastomere. 

 With the production of these eight cells the regularity of the 

 cleavage ends. The next step is the breaking up of the largest 

 blastomere [x) into three parts, two of which give rise to the 

 cells of the future mesoderm, and* are therefore designated as 

 mesoblasts, while the third, after dividing into a definite number 

 of parts, becomes the source of cells that are to form the 

 nervous system, and will therefore be spoken of as the primary 

 neuroblast. Rathke failed to understand this step, and Ixobiu 

 has fallen into the gravest errors and confusion with reference to 

 the same. First of all he states (-|4-j-) that the four ectoblasts 

 multiply to the number of eight by division, before the cleavage 

 of the dorsal segment begins. Robin has given essentially 

 correct drawings of the first division of x in the figs 240 — 251, 

 but that lie failed to understand it is proved by his letter- 

 designation ill lig. 2.51. The blastomercs and c (lig. 251) are 

 not the parts designated by these letters in fig. 250, but the 

 two parts that have arisen by the division of d ; and the jiair of 

 blastomeres marked dm fig. 351 correspond to a and c in fig. 



