gS CONK LIN. [Vol. XIII. 



in Nereis and Crepidula, this is the third division of the apical 

 quartette. 



It is very interesting to note that in so primitive a form as 

 Chiton, in which the cleavage in general appears to be very dif- 

 ferent from the ordinary molluscan type, the cross is present, at 

 least in some species. In Metcalf's ('93) figures of Chiton 

 marmoratus and C. squamosus the cross is found as in the 

 gasteropods just mentioned, being composed of cells of exactly 

 the same cell origin and position, except that the tip cell is 

 shown displaced a little to the right. These facts and several 

 others which will be referred to later are shown in Diagram 12, 

 e and/, which are copies of Metcalf's Figs. XIV and XXI V.^ 



Wilson ('92) has shown that in the polychaetous annelid 

 Nereis, a cross of ectoblast cells is present which in many 

 respects resembles the cross in Neritina, Umbrella, and Crepid- 

 ula. This annelidan cross, like the molluscan one, contains 

 two cells in each arm when first formed, and these later increase 

 to three. The centre of the cross, too, is formed by the four 

 apical cells. In its later history each arm of the cross under- 

 goes longitudinal splitting {cf. Wilson, Fig. 41), as is the case 

 in Crepidula ; but here the resemblances end. Professor Wil- 

 son has shown that the cross in Nereis differs both in origin 

 and destiny from the cross in Neritina, and that both differ 

 from Crepidula. This conclusion as to the difference between 

 the mollusk and the annelid I can only more fully confirm, 

 though, as I have already pointed out, it is almost certain that 

 Blochmann's derivation of th6 cross in Neritina is wrong, and 

 that it has the same origin, structure, and destiny in Neritina 

 and Crepidula ; the same thing is true of the origin and struc- 

 ture of the cross, at least in the early stages, in Umbrella. But 

 in neither origin, structure, nor destiny does the molluscan 

 cross resemble the annelidan. Wilson seems to consider them 

 alike in structure, for he says (p. 442) : " It is certain that, 



1 The cross is beautifully shown in Ishnochiton from the California coast, 

 which is being studied at present by Mr. Harold Heath in the Zoological Labora- 

 tory of the University of Pennsylvania. This work, which is far the most com- 

 plete yet done on Chiton, shows that the cleavage in that animal not only belongs 

 to the gasteropod type, but that it is, for a considerable period, cell by cell the 

 same. 



