ORGANIZATION AND CELL-LINEAGE OF ASCI 1)1 AN EGG. 59 



anterior and posterior quadrants is finally brought back to its right position and 

 given its true designation. In all stages later than his figure 60 the designation 

 A" stands for the same cell in Castle's figures and in my own. 



The changes in the designations of the cells which arc brought about by this 

 inversion of the orientation at the 48-cell stage may be most easily seen and appre- 

 ciated by a reference to the accompanying text figures XXV and XXVI, where the 

 designations of the cells, according to my interpretation, are given on the righl side 

 of figure XXV and on the left side of figure XXVI, while Castle's designations of the 

 corresponding cells are given on the Left side of figure XXV and on the right side 

 of figure XXVI. Barring the exceptions mentioned in the preceding paragraph. 

 Castle has followed with substantial accuracy the subsequent lineage of the dorsal 

 hemisphere up to a stage of about one hundred and twelve cells, though always 

 upon the basis of his erroneous lineage of the 48-cell stage. With the exception of 

 a single pair of cells, I need not further explain my departure from Castle's nomen- 

 clature of the later stages. This exception is the pair of small posterior mesen- 

 chyme cells which Castle designates C 7,5 , D 75 ; inasmuch as I find that they lie 

 ventral to their sister cells. I shall designate them B' 6 , B 76 , and their more dorsally 

 placed sister cells B 75 , J3 r ' 5 . 



With the completion of the sixth cleavage we reach a period when the gastru- 

 lation is ready to begin. Already preparations for the gastrulation are apparent in 

 the changing shapes of the cells of the dorsal and ventral hemispheres, in the rela- 

 tive positions of the cells and in the directions of their divisions. Even the pecu- 

 liar type of the chordate gastrula, with its overgrowing anterior lip and its nearly 

 stationary posterior one, is foreshadowed at a very early stage in the eccentric posi- 

 tion of the animal and vegetal poles in the two hemispheres of the egg. 



In the 32-cell and 64-cell stages it is apparent that the animal and vegetal poles 

 do not mark the middle of the ventral and dorsal faces of the embryo. This was 

 first noticeable in the 4-cell stage of Cynthia where the two posterior cells are 

 smaller than the anterior ones. In the 8-cell stage the anterior-ventral cells are 

 elongated antero-posteriorly. while the posterior-ventral ones are elongated trans- 

 versely: this brings the animal pole still farther back of the middle of the ventral 

 face. In the 16-cell stage there are two pairs of cells adjoining the mid-line in 

 front of the animal pole and but one pair behind it ; in the dorsal hemisphere there 

 is one pair of such cells in front of the vegetal pole and two behind, but the most 

 posterior pair is smaller than the others, so that the vegetal pole lies near the mid- 

 dle of the dorsal face. In the 32-cell stage three pairs of cells adjoining the mid- 

 line are in front of the animal pole, two behind it; while in the dorsal hemisphere 

 there are two pairs of such cells in front of the vegetal pole and two behind it. 

 though here again the most posterior cells are very small ones. In the 64-cell 

 stage (digs. 130, 131) there are four pairs of cells adjoining the mid-line in front of 

 the animal pole and only two pairs behind it; while in the dorsal hemisphere there 

 an- four pairs of such cells, both in front of and behind the vegetal pole, but the 

 most posterior pair are the tin\ mesenchyme cells (B 7 - 6 , B 7 - 6 ) which are partly cov- 



