194 PROCEEDINGS OF THE AMERICAN ACADEMY 



II. Fig. 10). There are indications, however, that they were re- 

 ally derived from the apical quartet a^-d-^ ; for (1) their nuclei 

 are nearer those of the apical quartet ; (2) the cells of the apical 

 quartet are much smaller after the cells a 2 u -d 2 11 appear than be- 

 fore ; (3) a 2 -d 2 have just arisen by a recent division, whereas 

 some time has elapsed since the first division of the apical quartet, 

 Figure 45 (Plate II. Fig. 7). If this interpretation holds, the cells 

 «! and d 2 11 result from the division of a x , while b r and a 2 u come 

 from b, etc. The inner part of the "cross" would thus arise 

 from the first group of micromeres and the outer part from the 

 third group. These suggested changes reduce the cleavage of 

 Neritina to complete agreement with that of Nereis, Umbrella, 

 and Limax. 



Wilson ('92, p. 439) compares the cleavage of Nereis with that 

 of the poly clad Discoccelis, and also with that of the gaster- 

 opods — taking Neritina and Crepidula as types — as follows : 

 " Up to a late stage in the spiral period (twenty-eight cells) ev- 

 ery individual blastomere and every cell division [in Nereis] is 

 represented by a corresponding blastomere and a corresponding 

 cell division in the embryo of the polyclad and in that of the gas- 

 teropod." This statement must imply, it seems to me, some 

 other interpretation of the cleavage of Neritina than that given 

 by Blochmann himself, although Wilson makes no statement to 

 that effect, but on the contrary says (p. 442), " It is impossible to 

 explain the differences between the annelidan and molluscan cross 

 by assuming inaccuracy of observation 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 2 - 2 , c 2 - 2 ) from which they 

 rise." The reduction of Neritina to agreement with Nereis and 

 Umbrella in the manner I have suggested brings it also into har- 

 mony with the law of alternation of spirals, and this affords an- 

 other presumption in favor of the correctness of the revision here 

 proposed. That the spirals do alternate in Neritina can be seen on 

 an examination of the short arrows indicating my interpretation 

 of the genetic relation of the cells of Neritina (Plate II.). The 

 arrows show the relation of the cells, the head of the arrow in 

 every case lying in the derivative which, according to my inter- 

 pretation, is the upper one. The following table will assist in the 

 comparison of the two interpretations and the determination of 

 the spirals. 



