The Development of Ischnochiton. 593 



flattening of the externa] surface, which takes place to such an extent 

 that at the time when the cells become ciliated they form a con- 

 striction about the egg and appear in marked contrast to the remaining 

 ectodermal cells. During this time the trochoblasts have decreased 

 their external, and in many cases increased their internal, surfaces. 

 In other cases the inner and outer surfaces are about equal and at 

 such times the cell appears barrel-shaped in section. 



In the interior of the cells after the cilia have become active the 

 cytoplasm shows imbedded in it droplets of various sizes that are 

 probably metabolic products formed as a result of the activity of the 

 cilia. These continue to accumulate for several days until the fluid 

 contents is in excess of the cytoplasmic, and at the free swimming 

 stage the cells become turgid and project above the general surface 

 of the body (Fig. 54). About two days after the embryo has escaped 

 from the chorion the cells, which have been gradually pushed out, 

 burst and are thrown away. For several hours a slight constriction 

 appears where the velum was located but after that time all traces 

 of the organ disappear. 



Comparisons. 



Among Molluscs the origin of the velum has not hitherto been 

 accurately determined, but in two Gastropods, Neriiiua and especially 

 Crepidîda, several facts are known which are of the highest interest. 



In the first form mentioned, Blochmann ('82) determined that the 

 right and left tip cells enter the velum. In thus becoming secondary 

 trochoblasts they correspond to the few carefully studied Molluscs, 

 and to several Annelids also, but how much farther the resemblance 

 may extend cannot at present be determined , since the development 

 of the remaining portions of the velum is unknown. 



In Crejndula, on the other hand, much more is known. The 

 two anterior trochoblasts, formed at the first division of the first 

 quartette, become velar cells. Whether the posterior cells are destined 

 to a similar fate has not been determined, but since there is a wide 

 posterior gap in the velum it may be that these cells remain non- 

 functional. 



Anteriorly, certain cells from the second quartette derived prob- 

 ably from the tip cell enter the velum, and there is a strong prob- 

 ability that the tip cells of the transverse arms also contribute. 

 These velar cells divide repeatedly and ultimately become relatively 

 small, yet it is obvious that the portion of the velum just described 



