410 GEORGE W. TANNREUTHER 



embryo and have more the appearance of yolk cells. These 

 cells gradually recede from the pharyngeal wall (fig. 5), and 

 finally take up a position on the dorsal side of the embryo (figs. 

 2, 6). Figure 6 represents the adult condition of the digestive 

 tract in the male at birth. The stomach in the female rotifer 

 Asplanchna ebbesbornii ends bhndly. There is no indication 

 of an intestine in the early stages of embryonic life. 



REPRODUCTIVE SYSTEM 



The origin of the reproductive organs is directly associated 

 with the entoderm and they arise from the derivatives of the 

 mesentoblast. Thus, the digestive and the reproductive organs 

 have a common origin. The differentiation of the reproductive 

 system is very rapid, especially in the male, since they are sexually 

 mature before birth. Practically all of the derivatives of the 

 large cells E^-^-^ and E2-i-2 of the five large mesentodermal cells 

 (fig. 67) contribute to the formation of the reproductive system. 

 About the first indication in the differentiation of the digestive 

 and reproductive regions is the appearance of darker granules 

 in the reproductive portion. This differentiation is well marked 

 during the early formation of the pharynx (figs. 76, 77, 79, and 

 87). These cells are indicated by heavy stippling. 



The reproductive cells continue to divide and later become 

 specialized into two groups or regions (fig. 90), one forming the 

 vitellarium and the other the oviduct and uterus, which becomes 

 continuous with the urogenital sinus (figs. 95 to 97). The 

 differentiating vitellarium is at first a spherical mass of cells, but 

 later sends out an arm right and left and finally becomes U-shaped 

 (figs. 95 to 104). The vitellarium produces the yolk and supplies 

 the egg with its granular and yolk content. Yolk production i s 

 well marked in those individuals that produce thick-shelled 

 resting eggs. A small portion of the vitellarium, in the region of 

 the oviduct becomes specialized into a rudimentary ovary (figs. 

 100 to 104), which contains a number of very minute cells. 

 These cells enlarge, one at a time (occasionally two develop 

 simultaneously), and become the mature ova. In the older 

 female, at the close of the reproductive period, the ovary is very 



