ASPLANCHNA EBBESBORNII (rOTIFER) 401 



During gastrulation the embryo shortens and increases in 

 width, as shown in figures 55 to 57. The formation of the central 

 cavity (figs. 57 and 58) and gastrulation occurs very rapidly. 

 The entire process requires about fifty minutes and can be 

 demonstrated in the living egg. Figures 58 and 59, a sixty-four- 

 cell stage, viewed from the dorsal and ventral sides, respectively, 

 show the embryo at the end of gastrulation. The blastopore, 

 situated at the macromere end of the embryo, is rather large at 

 first and is surrounded by eight cells, two belonging to each of 

 the four quadrants (figs. 52 to 54). 



Gastrulation brings about a fundamental change in the re- 

 lation of E to the remaining cells of the embryo. At first it 

 formed the posterior end of the cleavage cells, but at the end of 

 gastrulation it occupies the central region of the growing em- 

 bryo and is completely enclosed by the surrounding cells (figs. 58 

 and 59). The large cell E is now designated as the mesentoblast, 

 and, on account of its new position is destined to assume a new 

 r61e in the development of the embryo. The embryo at the end 

 of gastrulation contains about 200 cells, which are divided into 

 two distinct regions; the mesentoblast including d^ and d^, and 

 the epithelial ectoderm, which, at the anterior end, shows the 

 beginning of a double layer (figs. 58 and 59). 



SEGREGATION OF THE GERM LAYERS 



In Asplanchna ebbesbornii a distinct segregation of the germ- 

 layers begins with the formation of the cell d^ or d^, the nine- or 

 seventeen-cell stage. The cell d^ is not usually formed, hence 

 the variation in the number of cells at the time of segregation. 

 The large posterior cell D is destined to become entomesodermal, 

 and all of the remaining cells are ectomesodermal. 



1. Ectoderm 



The dorsal part of the ectoderm is derived from the quadrant 

 D, the right, the left, and the ventral parts from C, A, and B, 

 respectively. The three cells of the quadrant D, d^, d^, and d^^ 

 are the first to divide in the formation of the ectoderm during 



