442 



WILSON. 



[Vol. VI. 



of cells, which Conklin has also found in Crepidiila. The arms 

 of the cross meet at a right angle, in the centre of the upper 

 hemisphere, and lie directly above the four respective macro- 

 meres, as in Nereis — i.e. they are inclined at an angle of 45° to 

 the first two cleavage-planes (see Blochmann's Fig. 53). The 

 principal portion of each arm likewise consists primarily of three 

 cells, precisely as in Nereis. The resemblance is in fact so pre- 

 cise that at first I had no doubt the two were perfectly homolo- 

 gous. An exact comparison shows, however, that tJie cross-cells 

 have a totally dijfcrent origin in the two cases. In Nereis, the 

 four arms arise respectively from the four primary micromeres, 

 after the completion of their three spiral divisions, and the 

 remains of the four micromeres {a}, b^, c^, d^) persist as the pole- 

 cells at the extremities of the cross. The composition of the 

 arms is therefore, a^, a^-^, a^-^ ; d^, b^-^, b^-^, etc., the centre 

 being occupied by the rosette-cells, a^^, b^-^, etc. {cf. Figs. 37, 

 38). In Neritina, on the other hand (accepting Blochmann's 

 conclusions), the arms are derived from the second set of micro- 

 meres, the centre being occupied by the four primary micro- 

 meres. The arms of the cross in the two forms have in fact the 

 following composition as compared with one another (I use 

 throughout the nomenclature employed for Nereis, which differs 

 somewhat from Blochmann's): — 



Nereis 

 Neritina 



^■2,2,1,1 



,4 ^1 



:,2,1,1,1 _ A2,2,l 



^2,2,1.1, 



^1,4 _ etc. 

 ^2,2,1,1.1 _ etc. 



In other words, the cross-cells of Neritina correspond, in mode 

 of origin, to derivatives of the stomatoblasts and of the first 

 somatoblast in Nereis ! 



Professor Conklin informs me that his observations on Crepi- 

 diila do not quite agree with Blochmann's, as the outer portion 

 of each arm arises from the second set of micromeres, the 

 inner portion from the first group {i.e. from a)--'^, b^^, etc.). 

 This, however, agrees no better with Nereis than Blochmann's 

 account. 



It is impossible to explain the differences between the anne- 

 lidan and the molluscan cross by assuming inaccuracy of obser- 

 vation on Blochmann's part, since the pole-cells of the lateral 

 arms show a peculiar granulation that may be seen in the 

 parent-cells {a^-^, c^'^) from which they arise. It is certain that, 



