354 ZOOLOGY SECT. 



which, the archenteron opens freely into the shallow space that 

 lies between the yolk-endoderm and the yolk. It is from the 

 common cavity thus formed that the lumen of the enteric canal 

 is derived. At a somewhat earlier stage a thickening (pr. pi.) 

 has appeared in the yolk-endoderm in the region which will give 

 rise to the head of embryo. This is the protochordal plate ; it 

 enters into intimate relationship with the endoderm cells that roof 

 over the archenteron, and, when the floor of the latter becomes 

 opened out, forms with them a continuous plate. In this the 

 notochord originates along the middle line, and the mesoderm of 

 all the region in front of the blastopore grows out from it at the 

 sides. The aperture of invagination becomes narrowed, and is 

 eventually closed by the approximation and coalescence of its 

 edges. In the region in which the coalescence of the edges takes 

 place there is for a time complete union of the layers, as in the 

 region of the primitive streak of Birds and Mammals. The anterior 

 part of the aperture, however, remains open for a time as the 

 opening of the neurenteric canal. 



In front of the blastopore a longitudinal depression bounded by 

 a pair of longitudinal folds (Fig. 1017, med. f) is the beginning of 

 the medullary groove. As this becomes closed, it encloses, in its 

 posterior portion, the blastopore or dorsal opening of the neurenteric 

 canal. At the sides of the medullary groove appear the proto- 

 vertebrse (prot. v) : the general history of these parts has already 

 been sketched in the section on the Craniata, and further details 

 will be given in the account of the development of Birds, which 

 agrees with that of Keptiles in most essential respects. Under the 

 head of Birds also will be found an account of the formation of the 

 characteristic foetal membranes, the amnion and the allantois, 

 which applies in all essential respects to the Keptilia as well. 



A species of the genus Chalcides or Seps, a Lizard with vestigial 

 limbs, which is viviparous, is apparently exceptional in the forma- 

 tion of a structure closely homologous with the placenta of Mammals, 

 a structure by means of which an intimate connection is established 

 between the embryo and its membranes and the wall of the special 

 compartment of the oviduct in which it lies. As in the case of the 

 Mammal, the intimate union thus brought about facilitates the 

 transmission of nourishment from the blood of the parent to that 

 of the foetus. 



Ethology. The Lizards are, for the most part, terrestrial 

 animals, usually extremely active in their movements and endowed 

 with keen senses. The majority readily ascend trees, and many 

 kinds are habitually arboreal ; but the Chameleons are the only 

 members of the group which have special modifications of their 

 structure in adaptation to an arboreal mode of life. The Skinks 

 and the Amphisbsenians are swift and skilful burrowers. The 

 Geckos are enabled by the aid of the sucker-like discs on the ends 



