JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 95 
Lamcere (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 eleavage (Figs. 19-30, Plates 3 and 4), a 
romarkable difference is to be observed between tho 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 DO 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 desi oye 
linka) divides twice in succession before the fourth cleavage of any of 
the other cells. After these two divisions of d, 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 eleavages aro finished 
that the cell d+? (III, Zelinka) is separated into two blastomeres. Be- 
fore this division of d*? takes place, the egg consists in Callidina, as in 
Asplanchna, of four rows of four cells each, but in Callidina the method 
of origin of tho four cells of quadrant D is stated to be different from 
that in the other quadrants. In this quadrant tho 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 Molicerta, according to Zelinka, the cleavage up to this point is as 
in Asplanchna; the cell I (4*1) divides first, then II (d*?), then the 
cells of the other three quadrants. Later the cleavage of Melicerta 
differs from that of Asplanchna, but up to the end of the sixteen-cell 
stage the processes are the same in the two. 
In Fosphora, as described by Tessin (86), the cleavage is like 
that of Asplanchna, except in the unessential particular that his cell 
a’ (= d*?) divides before the cleavage of a (== dt). 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 deseribed in Callidina by 
Zelinka was due to defective observation. Zelinka has noted this point 
with particular attention, and states that he is certain of the difference 
