ORGANIZATION AND CELL-LINEAGE OF ASCI [MAX EGG. 53 



umnar, while the dorsal cells become relatively broad and lint (texl figs. XI. XII, 

 XIX. XX). This change of shape has been observed and commented upon bj 

 Van Beneden ami .lulin. Samassa, and Castle, and both of the latter authors attri- 

 bute the columnar form of the ventral cells to the pressure exerted upon them by 

 the overgrowth of the cells of the dorsal hemisphere; both regard this overgrowth 

 as the beginning of gastrulation (epibole). Whatever may he the cause of the 

 shapes of the cells at the two poles, whether purely mechanical or not. it is certain 

 that this is not the beginning of gastrulation, since, as I will show later, the 

 columnar cells of this stage become the flattened ectoderm cells of Later stages, 

 while the flattened cells of this stage become the columnar endoderm cells of the 

 gastrula. 



6. Sixth Cleavage; 32-64 <*lk. (Figs. 4:5-45, 120-130, 194-197). 



In this cleavage the divisions are not synchronous, the cells of the dorsal hemi- 

 sphere dividing before those of the ventral as in the preceding cleavage, and some 

 of the cells in the posterior half dividing later than those in the anterior one. 

 Accordingly it would be possible to sub-divide the period between the 32-cell and 

 (14 -cell stages into a 44-cell, a 46-celI and a 48-cell stage, as Castle does. These 

 stages, however, are of brief duration and all the cells of the sixth generation 

 divide before any of the seventh do; therefore, the sixth cleavage is distinct from 

 preceding and succeeding ones. 



The spindles appear in the four chorda-neural-plate cells at the anterior border 

 of the dorsal hemisphere in a nearly dorso-ventral direction. The four ventral pro- 

 duets of this division (A' 4 , A 78 ) form a band of small cells around the anterior bor- 

 der of the egg just dorsal to the equator; these cells ultimately give rise to the 

 posterior part of the neural plate; the dorsal products (A 7-3 , A 7 - 7 ) give rise to the 

 chorda (figs. 119-123). The neural plate cells are small and contain little or no 

 yolk, whereas the chorda cells are larger and are yolk-laden (text figs. XIX, XXI. 

 XXIII); this cleavage of these cells is therefore markedly differential. 



While these cells are dividing, all of the endoderm cells divide; these are the 

 four median cells which meet at the vegetal pole (A" 1 . A" 1 . IV' 1 . ]\ M ). and a single 

 pair of cells which lie lateral to these and in front of the transverse (second cleav- 

 age) plane (A 6-3 , A 6 - 3 ). The spindles in the median cells are antero-posterior in 

 direction, while those in the lateral cells are nearly transverse (figs. 120, 121, 123). 

 These divisions are equal and non-differential in the median cells; in the lateral 

 cells the division is differential, the inner product (A 75 ) being rich in yolk, the 

 outer (A 7 - 6 ) containing more protoplasm : the former is an endoderm cell, the latter, 

 according to Castle, mesenchyme. By these divisions ten endoderm cells are pro- 

 duced, live on each side of the mid-line, and two mesenchyme cells (figs. 44-46). 



While the divisions of the endoderm and chorda-neural-plate cells are occur- 

 ring, the most anterior mesoderm cell (B 02 , B 1 '-). forming the point of the crescent 

 on each side, divides, the spindle lying in a nearly dorso-ventral direction (figs. 43, 

 44, 120-129, 193, 194). This division, in fact, sometimes slightly precedes that of 



