68 J. F. McCLENDON. 



by this time the ectoderm has completely closed over them 

 (Fig. 42). This mass of cells divides in the sagittal plane into 

 two masses (Fig. 44), which begin to grow forward as a pair of 

 broad bands under the neural thickenings of the ectoderm (Figs. 

 46, 47, Mp]. From the anterior ends of these bands oval masses 

 are cut off that are the mesoblastic somites (Fig. 48). 



IV. ENTOBLAST. 



The entoblast is segregated one generation later than in Lepas. 

 In the 32 cell stage the entoblast consists of one cell that con- 

 tains practically all the yolk and which does not divide until the 

 majority of the cells have completed the seventh cleavage and 

 some are in the eighth. It then forms a very long transverse 

 spindle with the poles inclined anteriorly. The daughter nuclei 

 are widely separated, but in Pandams, Lannargiis and other 

 Caligidffi the yolk does not segment. The next (second) division 

 occurs about the time the "blastoderm" has covered half the 

 yolk. The spindles extend longitudinally and the poles are 

 inclined outward. Each of the two spindles is shorter than that 

 of the previous division (Figs. 35, 38). The next (third) division 

 occurs about the time of the closure of the blastopore (Fig. 40). 

 There is much variation in the direction and curvature of each of 

 the four spindles, but the daughter nuclei are about equally dis- 

 tributed through the yolk as they are after each division. The 

 fourth division occurs in stage B (Fig. 42) and the fifth in stage 



<^(Fig. 43)- 



In Eudactylina the yolk segments in the first three cleavages 

 of the entoblast, (forming eight cells) after which the entoblast 

 forms a syncytium. In the dichelestid the yolk divides into 

 four cells and is then transformed into a syncytium. In the 

 remaining species studied a syncytium is formed from the first. 

 This omitting of the cleavage of the yolk is probably not entirely 

 due to the amount of yolk present, which is as great in the 

 dichelestid as in Lczinargns, but largely due to the extent of 

 compressions of the egg, for it has gone farther in those eggs 

 which are compressed the most. The entoblast nuclei migrate 

 to the surface of the yolk and form the enteron or mid gut, as 

 described by Pedaschenko. 



