THE INDIVIDUALITY OF THE GERM-NUCLEI 



DURING THE CLEAVAGE OF THE 



FERTILIZED EGG OF THE 



ROTIFER, ASPLANCHNA 



INTERMEDIA. 1 



BARBARA WIGGENHORX, 

 DAVID D. WHITNEY. 



Recently while studying sections of the rotifer, Asplanchna 

 intermedia (Hudson and Goss), double nuclei were discovered in 

 the fertilized eggs while only one nucleus was seen in the parthe- 

 nogenetic eggs. All of the cells in the early cleavage stages 

 showed the two nuclei but in the later stages fewer and fewer 

 cells contained two nuclei. It was decided to study the sections 

 in an attempt to determine how late in the embryonic develop- 

 ment the two nuclei remain distinct. 



This species produces small parthenogenetic eggs developing 

 into males, larger parthenogenetic eggs developing into females 

 and large fertilized eggs which also develop into females. Both 

 kinds of parthenogenetic eggs may be distinguished readily by 

 their very thin covering membrane. It appears to be no thicker 

 than the covering membrane of the blastomeres of an embryo. 

 (Fig. i, A~G}. The covering of the fertilized egg is many times 

 thicker and also shows a different structure than that of the 

 parthenogenetic eggs. By noting the covering membranes alone 

 one can distinguish these two kinds of eggs at a glance. (Fig. I, 

 A, D, II). No observer has ever found a parthenogenetic egg 

 with a thick shell in this species. 



When the male parthenogenetic egg is fertilized it becomes 

 larger, develops this thicker characteristic shell and is popularly 

 known as the winter egg. The origin of both of these eggs is 

 identical but their mature appearance depends upon whether they 

 are fertilized or not. They both produce two polar bodies while 

 the female parthenogenetic egg only produces one polar body. 



Studies from the Zoological Laboratory, The University of Nebraska, No. 142. 



402 



