The Development of Ischnochiton 625 



which have undergone several divisions, lie either wedged in between 

 the secondary or pushed before these become located upon the macro- 

 meres within the blastopore. Their subsequent fate has not been de- 

 termined further than that they do not contribute to the formation 

 of the mesoblast. However in proving this point Wilson figures a 

 number of stages which show in the clearest manner that several of 

 the tertiary cells become enclosed within the circle of secondary 

 stomatoblasts, therefore within the limits of the blastopore, and that 

 very soon after this stage a small opening, the mouth, arises within 

 the circle and some of the third quartette cells form the roof of a 

 shallow archenteron. 



In comparing these tertiary cells with the third quartette stomato- 

 blasts of Ischnochiton it is seen that they arise and are located 

 similarly, and considering their relatively small size they have very 

 much the same development, and it appears very probable that these 

 cells, as the secondary are known to do, enter permanently into the 

 stomodaeum. 



Now that it has been seen that the formation of the stomodaeum 

 in Annelids shows many points of resemblance to that of Ischnochiton^ 

 and that there are strong reasons for believing that the second and 

 third quartettes enter into its formation, it is rendered more probable 

 that in its later development Crepidula follows along the same line. 

 Indeed I believe it will be found the rule among Gastropods and 

 Annelids that the stomodaeum forms from both second and third 

 quartette cells. 



IX. History of the Mesomeres. 



1. The Mesoblast. 

 The mesoblast arises from the posterior macromere in the 72 cell 

 embryo. Its first division occurs in the 113 — 149 cell stage, and is 

 generally bilateral although in many cases the spindle indicates a 

 slightly leiotropic cleavage (Fig. 28). The next division, affecting both 

 cells simultaneously, occurs normally in the stage shown in Fig. 32. 

 The spindles when fully formed are directed inward toward the centre 

 of the embryo and the resulting blastomeres are situated on either 

 side of the median line on the dorsal side of the parent cells, abutting 

 against the macromere D (Figs. 32, 37, 38). This cleavage is per- 

 fectly bilateral as is manifested not only in the position of the spindles 

 but in the movements of the centrosomes previous to division. In 



