52 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



Divisions begin in the ventral hemisphere before they are finished in the dor- 

 sal (figs. 118, 19] ). Tn the most anterior and posterior pairs of cells of this hemi- 

 sphere (a 5-3 , b 5-4 ) the spindles are nearly parallel with tlie median plane; in the two 

 remaining pairs of cells (a 5 - 4 and b 5-3 ) the spindles are oblique from posterior-ventral 

 to anterior-dorsal (figs. 118, 191). The division of the anterior pair of cells (a 53 , 

 tins. 1 IS, 119) gives rise to a couple of cells (a 6-5 , a'"') which lie just above the equa- 

 tor and in contact with the chorda-neural-plate cells of the dorsal hemisphere. 

 Later development shows that these cells form part of the anterior portion of the 

 neural plate; the only other cells of the ventral hemisphere which enter into the 

 formation of this plate are portions of the cells a 6r , which lie on the lateral borders 

 of the cells a 6,5 . All of the cells of the ventral hemisphere are of practically the 

 same size and constitution ; each consists of a superficial layer of protoplasm, in 

 which the nucleus lies, and a deeper layer of yolk, the cells of this pole being 

 decidedly protoplasmic as compared with those of the opposite pole. 



The result of this cleavage is the formation of sixteen cells in each hemisphere 

 which may be tabulated as follows : 

 Ventral hemisphere 



14 ectoderm cells, protoplasmic. 

 2 neural plate cells, protoplasmic. 

 Dorsal hemisphere 



6 endoderm cells, yolk laden. 



4 chorda-nerve 1 cells, yolk and protoplasm. 



6 mesoderm cells, yellow protoplasm or crescent substance. 



32 cells. 



At the (dose of this cleavage the cells of the ventral hemisphere are smaller in 

 superficial area than those of the dorsal hemisphere; when viewed from the ventral 

 pole the dorsal cells are seen around the entire periphery of the egg, except at a 

 point on the right and left sides where a single ventral cell (b liD , b 6-B ) occupies the 

 periphery; this is the only cell of the ventral hemisphere which can lie seen from 

 the dorsal pole (figs. 11!), 192, 193). A similar condition prevailed at the close of 

 the preceding cleaving (figs. 116, 117, 190), the only cells of the ventral hemi- 

 sphere which could be seen from the dorsal pole being b 5:i and b 5 - 3 . This condition 

 may be traced still farther back to the 8-cell stage (figs. 108, 1 L0, L84) where the 

 ventral cells are smaller than the dorsal ones and where the only portion of the 

 ventral hemisphere which lies below the general plane of the equator is that part 

 of each of the posterior-ventral cells (b 4,2 , b 4 - 2 ) which meets the anterior-dorsal cell 

 in the cross furrow (figs. 108, 184). 



At the close of the fifth cleavage the superficial area of the ventral cells is 

 smaller than at any preceding stage and that of the dorsal cells is greater; this is 

 due to a change in t lie shape of the cells, the ventral cells becoming long and col- 



1 Throughout this paper the cells which are to give rise to chorda, nerve, muscle and mesen- 

 chyme are, for the sake of brevity, frequently referred to as if they had already given rise to these 

 structures. 



