1907.] NATUHAL SCIENCES OF I'lII LADEI.I'IIIA. S'iS' 



The two siii;ill(M- Mastomcros, .1 and C, are approxiinatcl}' equal in 

 size (fifr. 6). while of the remaining two, one is significantly larger than 

 the other. The larger of these, i.e., the largest of the four cells, is the 

 one denoted by /). and lies on the posterior side of the egg. The next 

 largest cell, li. is anterior, while the two smaller cells, A and C, are 

 lateral in position. It thus hajipens that the larger of the two blasto- 

 nieres in the two-cell stage gives rise to the posterior and right blasto- 

 meres, /) and C, while the smaller forms the anterior and left blasto- 

 nieres, li and ,1. Thus the first cleavage separates the posterior and 

 right sides from the anterior and left. 



Tnnu) Cleava(;k — Four to I'^ioiit Celt>s. 



Th(- lliird cleavage is strongly dexiotropic. The resulting cells, la, 

 \h, ](• and 1(/, come to lie, when completely separated, in the furrows 

 between the macromeres (fig. 8). They retain this position only until 

 after the next cleavage. As has been found in the case of other spirally 

 cleaving eggs, the resulting position of the blastomeres here is not due 

 to surface tension alohe. The s])indles from the moment of the break- 

 ing of the nuclear membranes indicate clearly the direction the cleavage 

 is to take (comp. fig. 12). It is thus a phenomenon inherent in the cell 

 structure, although surface tension no tlouljt plays a part in giving the 

 blastomeres their final position. 



The divisions of the four l)lastomeres do not usually take place 

 synchronously. Lang gives the following rhythm for Discocaiis. The 

 largest cell (D) divkles first, next the anterior large cell {B). and after 

 these have divided the lateral cells A and C divide at about the same 

 time. In Planocera inquilina as a greneral rule the same rhythm holds 

 true, although there are often exceptions to it. The spindles are usually 

 i)resent in all four of the cells at the same time, but those of the two 

 larger cells are nearly always more advanced. The fact that in the 

 great majority of cases this rhythm holds true is exceedingly interesting. 

 Lang (84) was the first to ])oint out that there was such a constant 

 rhythm in the divisions of embryonic cells. Lang found that not only 

 did this succession hold good for divisions of the blastomeres where 

 there is an actual difference in size, but also that the descendants of 

 these cells divided in the same order (see quotation p. 526). 



Since that time many other observers have found a more or less 

 similar and constant rhythm in various animals, e.g., Lillie (95) in 

 Unio, Jennings (96) in Asplanchna, Child (00) in Arenicola, and Nelson 

 (04) in Dinophilus. In Unio, Arenicola and Dinophilus the order is 

 D, C, A, B; in Asplanchna it is D, C, B, A ; while in Discoccelis and 



