18 Art. 3.-Naolii(le Vatsn 



Based upon Zii;(;LKn,'s ohservatioiis Kiumülimi tried to explain 

 the e^^todieresis of tlie cteiiopliore egg ])V adding a few sul)8idiavy 

 assumptions to liis own theory of eell-division in general ])reviously 

 put forth ('99), viz., that (a) the nuclear fluid is present along the 

 axis of the egg; (h) at the expense of the nuclear fluid the mem- 

 brane gi'ows rapidly; (c) the " cleavage head " is a structure com- 

 parable to the centrosome, and (d) tlie rays radiating from tlie 

 " cleavage head ' ' contract and pull down the cleavage furrow to 

 the micromen^ pole. 



FiSGiiEL expresses his view of the probable existence of the 

 pole-rays, which function as in ordinary cases of cytodieresis ('08 

 p. G20 et àry.). 



My experimental study on the ctenophore egg makes it im- 

 possible for me to accept anv of the above three hypotheses for the 

 following reasons. If, as Zieulkr maintains, the cleavage is due 

 to the contraction of an elastic ring around the egg, the curling-up 

 of the cleavage furrow towards the maciomere pole after the 

 removal of the nucleated portion is a thing not easily accounted 

 for. Still more difficult is it to apply his view to the case in which 

 a new cleavage furrow is formed at right angles to the old. The 

 above two facts are also against Hitumuler's assumption. And 

 the fact that there are no rays radiating from the "cleavage head" 

 into the entoplasm makes his view untenable. It is certain that 

 the cleavage is not accomplished bv the contraction of i)ole rays, 

 as FiscHEL incidentally states, as is seen in the cases in which 

 the nucleated part is removed. 



In his paper on the development of Liuenjes mrirtinus, Conklix 

 puts forth the view that the unilateral cleavage of the cœlenterate 

 egg in general is at least in part due to the structure of the oöplasin 

 itself, that is, thin central entoplasm with a firmer peripheral layer 

 ('08 p. 167). This we have no reason to deny, yet how such 

 a structure is favorable to one-sided constriction is hard to 

 understand. When we come to study the unilateral cleavage of 

 the njicromeres of the ctenophore egg, which are almost entirely 

 made up of ectoplasm, it becomes doubtful how much influence 

 the origir.al structure of the ooplasm exerts on the ]H'i-forniance of 



