ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 33 



work of Seeliger, Samassa, and Castle, in distinguishing the dorsal and ventral faces 

 in the pregastrular stages. This is due to the fact that the cells at these two poles 

 are of somewhat similar shape, size and arrangement, as may he seen by referring 

 to figures 117. 118, 120, L24, 13(1 and 131 of this paper. However, the differences 



between these poles are so marked that there never need he any confusion regard- 

 ing them. 



(1) The most striking difference between the two poles is found in the fact 

 that at all stages of the cleavage and gastrulation one pole is rich in protoplasm, the 

 other rich in yolk. This is particularly noticeable in Cynthia and Molgula, but is 

 also true of Ciona, though in this genus the differences between the two poles are not 



a"6 



XVI 



Figs. XIII-XVI. Camera drawings of eutire eggs of Ciona intestinalu viewed as transparent objects ; 

 Figs. XIII and XV are seen from the posterior pole ; Figs. XIV and XVI from the left side. Figs. XIII 

 and XIV represent a 44-cell stage passing into a 62-cell stage; Figs. XV and XVI a 76-cell passing into a 

 110-cell stage. The position of the equator, the boundary between protoplasm and yolk and the segmen- 

 tation cavity are represented as in the preceding figures. In Fig. XIV the cells A7-4 and A? 8 are neural 

 plate cells, A?-3 and A" " chorda cells; in Fig. XVI the neural plate cells are A 8 ?, A 8 - 8 , A 8 ."5, A 8 -' 6 , while 

 the chorda cells are A 8 -s, A 8 - 6 , A 8 -'3, A 8 - 1 '!. The crescent cells (mesoderm) are B 6 -3, BM, B?-t, B?-3 (Figs. 

 XIII and XIVl and their dirivatives B7-5, &<>, B7 7, B?- 8 , B 8 -5, B 8 - 6 , BK B 8 - 8 (Fig. XV and XVI). The 

 other cells of the lower hemisphere are endodermal. In Figs. XIII and XIV the cells bordering the seg- 

 mentation cavity (seen in optical section) are of about equal height at the two poles; in Figs. XV and 

 XVI the cells at the loVer pole are columnar, those at the upper pole flattened. The polar bodies are 

 present exactly where they appear in the drawings. 



quite so marked as in the other genera named. This difference is so great that in 

 properly stained eggs one can always tell at a glance which is the yolk pole and 

 which the protoplasmic. In eggs stained in picro-haematoxylin the protoplasm 

 is red or light purple, the yolk yellow and the two poles are so unlike that 



5 JOUKX. A. X. S. I'HII.A.. Vol.. XIII. 



