The Development of Ischnochitoii, 579 



early stages. That an early differentiation occurs appears probable, 

 but the landmarks up to the 32 cell stage are of too uncertain a 

 nature to give any definite answer to the question, and hence I am 

 unable to state that this larger cell contains the material of the 1st 

 and 2nd somatoblasts, as in Nereis, Unio, Amphitrite etc. The spindle 

 that initiated this cleavage is fully half of the diameter of the egg 

 in length, while the one giving rise to the first polar body is one- 

 ninth the egg's diameter. 



2. Second Cleavage, 4 cells (Figs. 2 — 4). 



In about half an hour the second cleavage furrow appears at 

 right angles to the first and behaving in the same manner divides 

 the egg into four cells. Usually one of these products is slightly 

 larger than the others, and in the drawings I have oriented it so 

 that it corresponds to D in many other forms (Nereis, Unio, Crepi- 

 dula etc.) but as I have said no definite proof is at present forth- 

 coming that such is the case. 



A cross furrow appears that is especially well marked at the 

 animal pole, and on the vegetative pole one is present at right angles 

 to the first, but the cells forming it are simply in gentle contact. 

 This feature persists for a considerable period in many eggs, affording 

 an excellent means of orienting the blastomeres and relating the first 

 and second cleavage planes to the axes of the embryo. In Isch- 

 nochiton however in the transition from the four to the eight cell 

 stage it ceases to be definite enough to be used as a landmark, and 

 hence I am ignorant of the relation of the four cell embryo to the adult. 



I have not taken up the minuter processes of cell division, but 

 one matter calls for remark. Previous to the division of the cell I 

 have often noticed that the two centrosomes come to lie on opposite 

 sides of the nucleus, and the fibres radiating from them often indent 

 the nuclear membrane as in Fig. 8. The process is very similar to 

 that figured by Watase ('9Û) in his study of the Cephalopod cleavage, 

 and it appears to be characteristic of other Mollusca. 



III. The Formation of the Ectoblast. 



1. First Quartette of Ectomeres, 8 cells (Figs. 5. 6). 

 The next division gives rise in a dexiotropic direction to four 

 cells, the first quartette of ectomeres. The spindles which introduce 

 this cleavage have their upper ends inclined to the right, and usually 



