LATER DEVELOPMENT OF CHICK AND RABBIT 427 



dorsal ccelomic wall. This is the mesonephros or Wolffian body, 

 which is the first functional kidney of the embryo. 



The permanent kidney of the adult, the third to develop, 

 is the metanephros, and its mode of origin is slightly different from 

 that of the two preceding. Towards the end of the fourth day a 

 sac-like outgrowth starts from the Wolffian duct near its point of 

 entry into the cloaca. This grows forward as a tubular structure 

 on the inner side of the posterior cardinal vein and above the 

 mesonephros. The tube itself is the ureter, and it gives rise to a 

 series of complexly branched outgrowths, the future collecting 

 tubules. The actual secretory tubules are derived from the nephro- 

 tome tissue of somites 33-35, which accompanies the ureter as it 

 grows forward. The metanephros has therefore a twofold origin. 



Nervous System. 



The early development of the nervous system has already been 

 treated and its development outlined up to the stage when, after the 

 closure of the neural folds, the three primary divisions of the brain 

 had been marked out and eleven neuromeres indicated. Certain 

 points in the subsequent differentiation of the brain call for notice. 



At quite an early stage an outgrowth appears on each ventro- 

 lateral aspect of the anterior end of the prosencephalon, and it is 

 termed the primary optic vesicle, to mark the fact that it is the fore- 

 runner of the eye. Across the floor of the brain, between the two 

 vesicles, is a depression, the optic recess. The region in the middle 

 line passing forwards from this to the final point of closure of the 

 neuropore is the lamina terminalis, and although it is morpho- 

 logically the anterior end of the brain, as a result of the flexures it 

 becomes turned in a posterior and, finally, even a postero-dorsal 

 direction. While at first the vesicles are in wide-open communica- 

 tion with the cavity of the prosencephalon, their proximal ends 

 become closed in dorsally until they are reduced to two narrow tubes, 

 the optic stalks, with small apertures, one on each side of the optic 

 recess ; the future development of these will be considered separately 

 later. The first neuromere swells out markedly, and at 40 hours 

 has produced on the dorsal side a slight indentation between it 

 and the next neuromere. This is the velum transversum, and a line 

 from it to the optic recess marks the posterior limit of the telence- 

 phalon, which is therefore constituted by the first neuromere. Near 

 the end of the second day a pair of dorso-lateral outgrowths push out 

 from the telencephalon. These are the beginnings of the cerebral 

 hemispheres, and they expand rapidly dorsally, anteriorly and 

 posteriorly. They are hollow, and their cavities, the lateral or first 

 and second ventricles, are in open communication with the cavity of 



