ASPLANCHNA EBBESBORNII (ROTIFER) 395 



passage of cytoplasmic and yolk granules into the egg is visible 

 under a low power. When the egg reaches the end of its growth 

 period, it is separated from the ovary (fig. 4), and enters the 

 upper end of the oviduct, which, in reality, encloses the greater 

 part of the ovary. A single polar body is formed. In most 

 cases immediately after this maturation, the following egg begins 

 its growth. Two eggs may begin their growth at the same time, 

 but this is very unusual. In the case of the male-producing 

 eggs, two polar bodies are formed, the first of which often divides. 

 The origin, formation, size, and development of the female and 

 the male-producing eggs are identical (figs. 2 to 4). 



Sexual or resting ova: The origin and growth of the resting 

 eggs are similar to that of the parthenogenetic eggs. The vitel- 

 larium, however, is very dark from the presence of a rich supply 

 of yolk. The yolk passes directly from the vitellarium into the 

 growing egg. Immediately after maturation and fertilization, a 

 very thick inner shell is formed from the cytoplasm. The contents 

 of the double-shelled egg cannot be studied except in sections 

 In many of these resting eggs no inner shell membrane is formed. 

 They are about the same size as the thick-shelled eggs, but are 

 more transparent and contain less yolk. Their cleavage stages 

 can be followed without the aid of sections. The number of 

 resting eggs in the uterus at any one time varies from one to 

 eight. In case of the female or male-producing parent, there 

 may be as many as sixteen embryos in the oviduct and uterus 

 at the same time, ranging from the early cleavage stages to the 

 mature young (fig. 2). 



CLEAVAGE 



1. Designation of the cleavage cells 



The nomenclature adopted in the designation of the cleavage 

 cells is a modification of the system used by previous investigators 

 on cell lineage. The first four cells (macromeres) are designated 

 by the capital letters A, B, C, and D. The generations of micro- 

 meres (ectomeres) by the small letters a, b, c, and d. The first 

 index number indicates the generation to which the ectomere 



