JEXNINGS: DEVELOPMENT OF ASPLANCHNA HEEEICKII. 95 



Lameere ('90) and Leydig ('54), have given figures of the four-cell 

 stage of Asplanchna Sieboldii. Beyond this stage there are no other 

 published figures of the cleavage of any species of Asplanchna. 



In regard to the fourth cleavage (Figs. 19-30, Plates 3 and 4), a 

 remarkable difference is to be observed between the cleavage of As- 

 planchna and that of Callidina as described by Zelinka. In Asplanchna 

 the cleavage takes place up to (and beyond) this point with the greatest 

 regularity both as to direction of spindles and as to sequence. The first 

 and second cleavages are meridional, the third equatorial, and the fourth 

 again equatorial. The sequence of cleavage is in every case D, C, B, A 

 (see nomenclature of cleavage, page 16). In Callidina, according to 

 Zelinka, the rhythm and regularity of the process is destroyed after the 

 third cleavage by the remarkable circumstance that the cell d^-^ (I, Ze- 

 linka) divides twice in succession before the fourth cleavage of any of 

 the other cells. After these two divisions of c?*'\ the six cells of the 

 other three quadrants are said to divide in the same succession that 

 occurs in Asplanchna, while it is not until all these cleavages are finished 

 that the cell d*-^ (III, Zelinka) is separated into two blastomeres. Be- 

 fore this division of d^-"^ takes place, the egg consists in Callidiua, as in 

 Asplanchna, of four rows of four cells each, but in Callidina the method 

 of origin of the four cells of quadrant D is stated to be diff"erent from 

 that in the other quadrants. In this quadrant the three dorsal cells 

 (posterior, Zelinka) are said to arise by successive cleavages of the 

 large ventral (anterior, Zelinka) cell, while in each of the other three 

 quadrants the four cells arise by the halving of the two cells previously 

 present. 



In Melicerta, according to Zelinka, the cleavage up to this point is as 

 in Asplanchna; the cell I (c^*'^) divides first, then III (c?^**), then the 

 cells of the other three quadrants. Later the cleavage of ]\Ielicerta 

 differs from that of Asplanchna, but up to the end of the sixteen-cell 

 stage the processes are the same in the two. 



In Eosphora, as described by Tessin ('8G), the cleavage is like 

 that of Asplanchna, except in the unessential particular that his cell 

 a" (= d^-^) divides before the cleavage of a (= d^-^). The sixteen-cell 

 stage is reached by the cleavage of the same cells as in the two species 

 of Asplanchna and in Melicerta. 



In view of the regularity of the cleavage in these four forms, one 

 might be led to suppose that the irregularity described in Callidina by 

 Zelinka was due to defective obsei'vation. Zelinka has noted this point 

 with particular attention, and states that he is certain of the difference 



