340 



PROCEEDINGS OF THE ACADEMY OF 



[April, 



blasts in Crepidida and Fiona is the direct result of epibolic gastrulation 

 in the one case, embohc in the other, which is in turn caused by the 



quantity and nature of the yolk which the 

 macromeres contain. An intermediate 

 condition is found in Nereis (Wilson, 

 1898). Text-figure 1 (a) shows a sagittal 

 section through the cleaving egg of Crepi- 

 dula after one enteroblast has been sepa- 

 rated from the mesoblast. The ectoblast 

 has here but half covered the yolk, and 

 the entoblastic element is thrown down- 

 ward and backward in the direction in 

 which it must go if it follows the ecto- 

 derm over the yolk, and finally reaches a 

 position posterior to the blastopore as 

 that structiu-e is closing (Conklin's fig. 

 61). In Nereis, text-figure 1 (b), the ec- 

 toderm has advanced much farther over 

 the yolk when the enteroblasts arise, and 

 here we see that these elements are also 

 directed downward but at the same time 

 anteriorly. The next and last step in 

 their change of position is illustrated by 

 Fiona, text-figure 1 (c), in which, on ac- 

 count of its invaginate gastrula, the en- 

 teroblasts are not only anteriorly directed, 

 ]3ut also at first lie higher than the cells 

 from which they arose. 



In Trochus (Robert, 1903) the meso- 

 blast arises at about the sixty-four-cell 

 stage by a Iseotropic division which sepa- 

 rates the very large cell 4d from 4D. 

 This cell divides dexiotropically and 

 equally when eighty-nine cells are present. When there are one hun- 

 dred and eighteen cells, each of the two derivatives of 4d divides, and 

 of the resulting four cells the anterior pair are the smaller. Later 

 the two larger posterior cells divide. Robert has not found endo- 

 dermal elements to arise from 4d,'_but does not reject the possibility 

 of such a condition. 



As might be expected from their close relationship, a nearer corre- 

 spondence in the cleavage series is found when we compare Fiona with 



Fig. 1. — Sagittal sections 

 through the gastrute of 



(a) Crepidula (Conklin). 



(b) Nereis (Wilson) and 



(c) Fiona. The entero- 

 blasts are lined, the meso- 

 blastic cells stippled. 



