156 CONKLIN. [Vol. XIII. 



the archenteric cavity, the roof of which is formed by the mac- 

 romeres, and the sides by the cells of the fourth and fifth quar- 

 tettes. During the process of gastrulation there has been no 

 real invagination of the macromeres, though they have been 

 greatly reduced in size by the formation of the fifth quartette 

 and have also changed their shape, growing broader at the 

 apical pole and narrower at the oral, and finally thinner in the 

 direction of the egg axis, so that they seem to have been 

 invaginated ; but this, of course, could not occur, since there 

 is no segmentation cavity. 



In later stages a cell, which is almost certainly 5D, moves 

 around to the ventral side of the archenteron, as shown in Fig. 

 6^, where it is seen dividing. By the movements of this cell 

 the archenteric cavity, and in fact all the parts of the embryo 

 posterior to the mouth, are given a distinct torsion. The prod- 

 ucts of this cell form the floor of the archenteron posterior to 

 the mouth. 



3. The Fozirth Quartette. 



Before the fifth quartette is completed by the formation of 

 5D, the cells 4 A, 4B, and 4C divide equally into 4A1 and 4A2, 

 4B1 and 4B2, 4C1 and 4C2, Figs. 58, 59. The spindles in 4A 

 and 4C are nearly horizontal so that the two cell products 

 lie at approximately the same level ; 4B divides in a dexio- 

 tropic direction, and the cell products afterwards shift position 

 so that one partly overlies the other. Fig. 60. This cleavage, 

 like that by v/hich the fifth quartette is formed, is neither typi- 

 cally bilateral nor spiral, but intermediate between the two. 



The cells thus formed lie close around the blastopore and 

 form the floor of the anterior portion of the archenteron. 

 Ultimately they contribute largely to the formation of the 

 small entoderm cells at the inner end of the stomodaeum. 



4. The E7iteroblasts. 



The four enteroblasts can be followed up to Fig. 65 without 

 a break. They lie in the mid line just behind the blastopore 

 and immediately under the ectoderm, and in all cases are closely 

 pressed together in a characteristic grouping. In Fig. 60 one 



