XIII PHYLUM CHORDATA 365 



position an aperture is formed through 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 (pi'.pl.) 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 contin- 

 uous 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. 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. 1006, 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 neuren- 

 teric canal. At the sides of the medullary groove appear the 

 protovertebrse {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 Reptiles 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 Reptilia 

 as well. 



The genus Seps, one of the limbless Lizards, which is viviparous, 

 is quite exceptional in the formation of a structure closely homo- 

 logous 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 ai-e habitually arboreal ; but the Chamseleons are the only 

 members of the group which have special modifications of their 

 structure in adaptation to an arbore.al 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 of their toes to run readily over vertical or overhanging 

 smooth surfaces, A few, on the other hand (Water-Lizards), live 



