XV 



NEMATODA 



447 



the dorsal surface. These at first form two rows interdigitating 

 with each other, but they quickly rearrange themselves so as to form 

 a single row of band-like cells. Miiller has observed two large clear 

 cells lying on the ventral surface in the anterior part of the animal. 

 These he surmises to be the mother cells of the excretory tubes. 

 Although these tubes extend in the adult through nearly the entire 

 length of the animal, they are each composed of a single hoUowed- 

 out cell. 



gen. gen. 



Fig. 351. — Transverse sections of embryo of Ascaris megalocephala, in order to show the 



invagination of the anterior mesoderm cells and of the mother cells of the genital organs. 



(After Boveri.) 



A, soction of a stage in which the endoderm cells are invaginated, but in which the mesoderm cells 



are still superficial. B, section of a stage in which the mesoderm cells have passed into the blasto- 



coelie cavity. C, section of a stage in which the genital cells are beginning to be invaginated. D, 



section of a stage in which the invagination of the genital cells is complete. 



When the young worm finally hatches out — which, under normal 

 circumstances, must occur amongst the horse-dung which has been 

 dropped on the ground — it is known as a Rhabditis larva, and 

 it only becomes converted into a mature worm after it has been 

 swallowed by a horse. 



The reader will not fail to observe what large gaps there are 

 in the account of the development which has just been given, and 

 what a field there is for future work. The problem of tracing the 



