STUDIES ON ASPLANCHNIA EBBESBORNII. 199 



B. CLEAVAGE AND GASTRULATION. 



It is not my intention to give a detailed account of the different 

 cleavages. A brief account of the more important stages of 

 development, as they occur in the parthenogenetic female and 

 male will be considered for the purpose of comparison in empha- 

 sizing the varying degrees of development and degeneration in 

 these organisms. 



The development of Asplanchnia ebbesbornii has not been 

 worked out by previous -investigators. Complete development 

 (except of the double-shelled resting egg), occurs within the 

 oviduct and uterus of the parent. The animals are extremely 

 transparent so that it is possible to make a study of the various 

 cleavage stages as they occur w T ithin the living body. The eggs 

 are extremely plastic with a well-defined vitelline membrane 

 (Figs. 5 and 6). The nuclear activities are distinct in the living 

 cells, and the order, rate and direction of the cleavage spindles 

 can be followed without any difficulty. Preserved material if 

 stained, dehydrated, cleared and mounted in damar immediately 

 after fixation, serves as a fine check in the study of cleavage. 

 The cell lineage of Asplanchnia Herricki, from early cleavage to 

 late gastrula, was worked out by Jennings. His entire work was 

 based upon the study of preserved material. The rotifers were 

 fixed in toto, the eggs dissected out and studied under the 

 cover slip. With few exceptions he was unable to verify the 

 presence of a distinct egg membrane. He seldomly found more 

 than a single embryo in the same uterus. 



The cell lineage of Asplanchnia ebbesbornii, for the most part is 

 a confirmation of Jennings' results. The eggs are somewhat 

 elliptical with the cleavage nucleus nearer the anterior end 

 (A-C, i). The first cleavage plane passes through the region of 

 the polar body or bodies and divides the egg into two very un- 

 equal cells (A-C, 2). The second cleavage occurs at an angle 

 of about forty-five degrees to that of the first, dividing the anterior 

 smaller cell equally and the posterior larger cell unequally 

 (A-C, 3 and 4). The four macromeres show considerable uni- 

 formity in the condition of their cytoplasmic structures. The 

 region immediately surrounding the nuclei is more transparent 

 and free from cytoplasmic granules. In the third cleavage the 



