278 



ZOOLOGY 



SECT. 



fresh-water fishes, in which there is also a ciliated larval stage, the 

 young animals do not become sexually mature until two of them 

 have permanently united with one another. 



Temnocephala produces relatively large eggs, stalked or sessile, 

 with a thick chitinous shell, enclosing a single oosperm and a mass 

 of yolk-cells, which later become fused into a continuous mass. 

 Segmentation results in the formation of an irregular, massive 

 blastoderm composed of cells of several sizes. In this collects 

 a rounded group of larger cells, in the middle of which a space 

 appears (Figs. 219, 220). The space (endoccele) increases greatly 

 in size, the boundary cells becoming spread out to form a thin 

 layer, and approaches the periphery of the egg. A part of the 



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Fig. 219.— Section through the blastoderm of Temnocephala, showing early stage of 



endoccele (en). 



endoccele becomes subsequently rounded off to form the internal 

 lining membrane of the pharynx, the cavity of which remains cut 

 off from the exterior by a partition, which is the outer part of the 

 wall of the endoccele, until the young worm is ready to leave the 

 egg. A short prolongation backwards from this cavity ends blindly 

 in a mass composed mainly of yolk, still containing degenerate 

 nuclei and cells which have wandered into it from the blastoderm. 

 Only at a late stage, when the yolk has become taken up, is an 

 arrangement of cells recognisable in the form of an intestinal 

 epithelium (endoderm) enclosing a lumen (intestinal lumen). An 

 ectoderm likewise does not appear as an embryonic layer, the 

 epidermis only appearing late, and extending over the surface 

 evidently by modification in situ of cells that have become 



