390 CELL-DIVISION AND DEVELOPMENT 



tion of division even at a very early period. In the echinoderms, for 

 example, it is well established that division suddenly pauses, or changes 

 its rhythm, just before the gastrulation (in Synapta at the 512-cell 

 stage, according to Selenka), and the same is said to be the case in 

 Ainpliioxns (Hatschek, Lwoff). In Nereis, one of the blastomeres on 

 each side of the body in the forty-two-cell stage suddenly ceases to 

 divide, migrates into the interior of the body, and is converted into a 

 unicellular glandular organ. ^ In the same animal, the four lower cells 

 (macromeres) of the eight-cell stage divide in nearly regular succes- 

 sion up to the thirty-eight-cell stage, when a long pause takes place, 

 and when the divisions are resumed they are of a character totally 

 different from those of the earlier period. The cells of the ciliated 

 belt or prototroch in this and other annelids likewise cease to divide 

 at a certain period, their number remaining fixed thereafter.^ Again, 

 the number of cells produced for the foundation of particular struc- 

 tures is often definitely fixed, even when their number is afterward 

 increased by division. In annelids and gasteropods, for example, the 

 entire ectoblast arises from twelve micromeres segmented off in three 

 successive quartets of micromeres from the blastomeres of the four- 

 cell stage. Perhaps the most interesting numerical relations of this 

 kind are those recently discovered in the division of teloblasts, where 

 the number of divisions is directly correlated with the number of seg- 

 ments or somites. It is well known that this is the case in certain plants 

 {CJiaracecB), where the alternating nodes and internodes of the stem 

 are derived from corresponding single cells successively segmented 

 off from the apical cell. Vejdovsky's observations on the annelid 

 Dendrob(2na give strong ground to believe that the number of meta- 

 merically repeated parts of this animal, and probably of other anne- 

 lids, corresponds in like manner with that of the number of cells 

 segmented off from the teloblasts. The most remarkable and accu- 

 rately determined case of this kind is that of the isopod Crustacea, 

 where the number of somites is limited and perfectly constant. In 

 the embryos of these animals there are two groups of teloblasts near 

 the- hinder end of the embryo, viz. an inner group of mesoblasts, from 

 which arise the mesoblast-bands, and an outer group of ectoblasts, 

 from which arise the neural plates and the ventral ectoblast. McMur- 

 rich ('95) has recently demonstrated that the mesoblasts always divide 

 exactly sixteen times, the ectoblasts thirty-two (or thirty-three) times, 

 before relinquishing their teleoblastic mode of division and breaking 

 up into smaller cells. Now the sixteen groups of cells thus formed 

 give rise to the sixteen respective somites of the post-naupliar region 

 of the embryo {i.e. from the second maxilla backward). In other 



1 This organ, doubtfully identified by me as the head-kidney, is probably a mucus-gland 

 (Mead). '■^ Cf. Fig. 171. 



