304 ZOOLOGY 



SECT. 



Ascaris, species of Filaria and others) are viviparous. Segmentation 

 is complete and usually unequal. In Ascaris the ovum divides first 

 into two cells, a dorsal and a ventral. The former becomes sub- 

 divided into two anterior and posterior and of these the latter 

 undergoes further division into two a right and a left. The ventral 

 cell of the two-celled stage gives rise to a median row of four cells 

 of which the second becomes differentiated as the primordial 

 endoderm cell, which gives rise to the entire endodermal layer of 

 the mesenteron. The rest of the cells of this stage go to form the 

 ectoderm, the mesoderm and the sexual cells. A segmentation- 

 cavity is sometimes present, sometimes absent. The descendants 

 of the original dorsal cell undergo repeated division to form a layer 

 of numerous small cells (primitive ectoderm) that grows over the 

 rest, bringing about a kind of epibolic gastrulation. Of the ventral 

 row of four cells the most anterior divides into two pairs of cells 

 destined to give rise to the mesoderm bands as well as the ectoderm 

 of the stomodseum ; the second, as already stated, is the primordial 

 endoderm cell ; the third divides transversely into two primordial 

 sexual cells, and the last divides into four caudal cells. These 

 last undergo rapid multiplication and give rise to the secondary 

 ectoderm, which pushes forwards the primary ectoderm as it spreads 

 over the surface and eventually covers the entire embryo as the 

 integumentary layer, the primitive ectoderm, now sunk beneath 

 the surface, probably going to form the nervous system, and some 

 cells given off from it contributing to the formation of the mesoderm. 

 The two mesoderm bands divide each into two dorsal and ventral 

 and produce the muscular layer of the body- wall. In certain 

 viviparous forms the elongation of the embryo into the vermiform 

 shape is preceded in the uterus of the mother by a flat disc-like 

 stage, and the gastrulation is completed by the curving downwards 

 and inwards of the thin edges of the disc until they come in contact 

 and fuse in the middle line ventrally. 



Many of the Nematoda have a curious and complex life- 

 history : a few examples will be selected for description. 



Rhabdonema nigrovenosum lives, in the sexual condition, in the 

 lungs of Frogs and Toads : it is remarkable among members of 

 the class in being hermaphrodite. The eggs are laid and the 

 embryos pass from the lungs into the enteric canal of the host, are 

 expelled with its faeces, and develop in water into a sexual 

 Nematode, called the Rhabditis-iorm, in which the sexes are 

 separate : in this the fertilised eggs develop in the body of the 

 female, and, when fully formed, make their way through the wall 

 of the uterus and proceed to devour the whole of the maternal 

 tissues, leaving nothing but the cuticle. Being set free, they live 

 in mud until they succeed in gaining access to a frog's mouth, 

 when they pass into the lung, develop hermaphrodite reproductive 

 organs, and so re-commence the cycle. It will be seen that we 



