CHAPTER VII. 

 NEMATHELMINTHES. 



Systematic : I. Nematoda s. str. 



II. GORDIIDJ:. 



I. NEMATODA S. STR. 



Embryonic Development. 



The eggs of the Nematoda, which are usually oval, but 

 occasionally spherical, are laid at very different times. 

 Sometimes they are deposited very early, even before 

 cleavage begins, and then are surrounded by a thick shell 

 {Ascaris lumhricoides, Trichocephalus dispar), whereas thin- 

 shelled eggs begin their development when still in the 

 parent, and may even continue to develop here to a quite 

 advanced stage. Still other Nematodes, as, for example, 

 Trichina spiralis and some species of Ascaris, are viviparous. 

 The embryonic development of a number of forms is known, 

 though, it is to be regretted, not perfectly. As far as ascer- 

 tained, the cleavage appears in general to be fairly alike in 

 all cases. It is total and approximately equal, and leads to 

 the formation of a blastula, which, to be sure, may be some- 

 what variously shaped. It may have the form of a mere 

 cluster of cells, designated by Goette as a sterroblastula 

 (Bhabditis nigrovenosa), or it may be a true vesicle, with, 

 however, only a very small cavity (Ascaris megalocephala), 

 or, finally, it may appear in the form of a bilaminar plate of 

 cells {Cucullanus elegans). 



At a very early period the fundaments of the germ-layers and the 

 differentiation of the various regions of the body can be recognized on 

 the segmenting egg (Goette, Hallez). As early as the first cleavage the 



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