The Development of Ischnochiton. 599 



Turning to Gastropods we find in Heymons' work on Umhrella 

 that the first division of the first quartette gives rise to the trocho- 

 blasts, and that the second forms the apicals. The resulting stem 

 cells (intermediate cells of Annelids, basal cells of Gastropods) are in 

 contact with the tip cells of the second quartette, and consequently 

 a well marked dexiotropic cross is present similar in all essential 

 particulars to the one in Ischnochiton. The trochoblasts now cleave 

 into two cells in each quadrant but the so called basal cell does not 

 give rise to a cell lying in the arms of the cross but cleaving leio- 

 tropically it forms a small cell lying in contact with the primary 

 trochoblasts (Fig. C, b). In this regard and in the origin, 

 behavior and arrangement of the cells we have the same 

 state of affairs that exists in Ischnochiton. The later stages 

 of the cross in umhrella are not known save that it is said the basal 

 and tip cells divide, but neither figures nor descriptions give any data 

 for comparison. However, enough has been determined to make it 

 certain that the crosses of Ischnochiton., the Annelids and Umbrella 

 in their earlier stages at least are homologous structures. 



In Crepidula, as Conklin has shown, a well marked cross exists 

 whose history has been followed much farther than in any other form. 

 Superficially it appears in its earlier stages almost identical with what 

 occurs in Ischnochiton. Closer examination however makes it apparent 

 that although the two crosses have the same position, the same general 

 appearance and for a considerable distance at least much the same 

 history yet they differ in origin. In both, the first and second 

 divisions of the first quartette give rise to trochoblasts and apicals 

 respectively, but while the third division is leiotropic in Ischnochiton, 

 the Annelids and Umhrella, and therein fulfils the law of alternating 

 cleavages as proposed by Kofoid ('95), it is dexiotropic in Crepidula. 

 This is the only well marked case of reversed cleavage in all quadrants 

 that is to be found in Crepidula up to the 60 cell stage and it is 

 of very great interest and importance to discover if possible why this 

 reversal occurs. 



In Umhrella eight trochoblasts form when there are 12 cells in 

 the cross ; in Amphifrite there are 16 trochoblasts and probably about 

 24 cells in the cross ; in Clymenella the same ; and as Conklin has 

 stated there are 28 cross cells in Nereis when 16 trochoblasts are 

 formed, while in Crepidula there are 42 cross cells while 6 or possibly 

 8 velar are present. All of which goes to show that while the develop- 

 ment of the cross and consequently the velar field is relatively rapid 



Zool. Jahrb. SU. Abth. i. Morph. 39 



