PLASMA-STRUCTURE IN EGG OF HYDRACTINIA « 217 



(fig. 63). The majority of the eggs cleave so that the different 

 materials are equally distributed in the two blastomeres; often, 

 however, the distribution is unequal. Complete separation of the 

 kinds of material may occur, as when the cleavage plane comes in 

 through the protoplasmic layer between the ends (figs. 63, b; 65, g; 

 66, a, b, e, /). Sections show this to be due to the position of 

 the nucleus in regard to the stratification. Since the eggs are 

 not centrifuged sufficiently to move the nucleus, it may lie in 

 any relation whatever to the egg-materials. It is, however, usually 

 found in the protoplasmic layer (fig. 64, a, d) in which case the 

 egg divides so as to distribute the materials equally (fig. 63, /). 

 The nucleus may also lie in the yolk-end of the egg (fig. 64, c) in 

 which case the cleavage-plane comes in through the yolk (fig. 

 63, a). The nucleus not infrequently appears at one side of the 

 protoplasmic layer (fig. 64, d), in which case the first cleavage- 

 plane separates the two kinds of material, one blastomere receiv- 

 ing in addition to part of the protoplasm, yolk and mitochondria 

 and the other the oil mass. 



Individuals showing various distributions of the materials 

 were isolated and their development was followed, apparently 

 normal swimming larvae resulting (fig. 65). The yolk and 

 mitochondria are confined to one region of the resulting planula, 

 the oil to another. Also eggs which cleaved so that the different 

 materials segregated in the two blastomeres were cut apart, sepa- 

 rating the blastomere containing yolk and mitochondria from the 

 one containing oil (fig. 66) . These were isolated and the develop- 

 ment followed, and although many died, I succeeded in getting a 

 considerable number of these half larvae. If the protoplasmic 

 area were nearly evenly divided between the two blastomeres, 

 both were apt to live. So far as I could tell those that lived were 

 perfectly normal, except in size, the one being small and white 

 from its oil content, the other large and greenish from the yolk 

 content. Both were ciliated planulae. Sections of these planulae 

 which were killed in Meves' killing fluid and stained with Benda's 

 method, show the mitochondria apparently unchanged in one, 

 while the other contains none. It is apparent, then, that up to 

 this point of development the mitochondria are not essential for 



