390 GEOEGE W. TANNREUTHER 



The morphology of the Rotatorian family Flosculariidae was 

 worked out by T. H. Montgomery ('03) in considerable detail. 

 Zelinka ('92) published an account of the development of Calli- 

 dina ruseola. Jennings ('96) traced the cell hneage of Asplanchna 

 herricki, for a few generations, but did not correlate the cells of 

 the early embryo with the adult structures. 



Some rotifers are very transparent; this makes possible, 

 not only a detailed study of the position and relation of the 

 various structures, but an investigation of the origin, growth, 

 and cleavage of the eggs. Where complete development occurs 

 within the parent, a sequence of the different stages from the 

 first appearance of the egg to the time of birth can be followed 

 with a considerable degree of accuracy. The extreme trans- 

 parency of the adult Asplanchna ebbesbornii, makes possible 

 a detailed study of the successive stages in development. 



The more important points in development may be summarized 

 as follows: 



1. The cleavage is unequal and regular. A small cleavage 

 cavity is present. The gastrula is formed by a modified epiboly. 

 The blastopore occurs at the posterior end of the developing 

 embryo. 



2. The first cleavage plane is at right angles to the future 

 median longitudinal axis of the adult. It divides the egg into 

 two very unequal cells. The smaller cell is ectodermal and 

 mesodermal, while the larger cell contributes to the three germ 

 layers. 



3. The gastrula consists of an outer layer of epithelial cells 

 enclosing an inner cell mass. The outer layer includes all of 

 the derivatives of A, B, and C and the three cells d^, d^, and d^ 

 derived from D. The inner cell mass includes all of the remain- 

 ing cells. 



4. The outer layer or ectoderm gives rise to the cuticle, hypo- 

 dermis, brain, excretory system, trochal disc, cilia, buccal 

 pouch, and musculature. The inner cell mass derived from 

 3D produces the remaining parts of the digestive system and 

 glands, the reproductive system, and a few of the muscles which 

 control it. 



