1904.] NATURAL SCIENCES OF PHILADELPHIA. 345 



to form the enteron has already begun by the depression of the smaller 

 cells which lie in the center of the vegetative pole, while the small cells, 

 E\ E^, e\ e^ at the anterior end of the teloblasts have become drawn 

 into the posterior region of the invagination (except for some varia- 

 tion, an instance of which is shown in fig. 72), where at this time they 

 help to close that portion of the gastral pit. As the primary enteric 

 cells sink into the cleavage cavity the small cells, E\ E\ e\ e\ come 

 into close connection with the posterior edges of 5C, 5D, 4a. Thus a 

 more or less complete cup-like invagination is brought about among 

 the entomeres, in which the smaller elements lie at the bottom with 

 the larger (4a2, 4b^ 4c2) between, and the small cells which have arisen 

 from these latter lying peripheral to them. Alwve, toward the ven- 

 tral surface, lie small cells of the second and third quartets around the 

 blastopore opening. 



In the formation of the enteric cells the manner in which the fourth 

 quartet arises appears to be characteristic of a number of Opistho- 

 branchs. This quartet is in Umbrella (Heymons, 1893), Aplijsia 

 (Blochmann, 1883; Carazzi, 1900) and Tethys (Viguier, 1898), as well 

 as in Fiona, larger than the macromeres remaining at the center of 

 the vegetative pole. 



The further development of the enteron will be discussed later. 



Cleavage History of the Ectomeres. 



As has been seen, the ectoblast arises immediately after the four- 

 cell stage by the three successively alternating cleavages in which the 

 macromeres participate, giving rise respectively to the First, Second 

 and Third Quartets of micromeres. The cleavage history of these 

 cells will now be taken up and their ultimate fate, as far as can be 

 determined, considered. 



The First Quartet. 



The formation of the ''turrets," la--ld-, and the ''apicals," la^-ldS 

 leading to the radially symmetrical twenty-four-cell stage, has already 

 been considered. Shortly afterward the apical cells divide in a dexio- 

 tropic direction, thus alternating with the preceding cleavage, and by 

 this division the four "basal" cells of the ectoblastic cross arise, while 

 between these and the central point of the egg lie the four small apical 

 cells from which they were derived (fig. 23). Before this cleavage had 

 occurred the upper and dextral cells of the second quartet had in each 

 quadrant given off a small cell in a Iseotropic direction (fig. 21), which 



