208 



GENERAL DEVELOPMENT. 



...am 



FIG. 129. SURFACE VIEW 



OF A YOUNG EMBRYO OF LA- 

 CERTA MURALIS. 



am. amnion 

 streak. 



pr. primitive 



General development of the Embryo. 



The formation of the embryo commences with the appear- 

 ance of the medullary plate, the sides 

 of which soon grow up to form the 

 medullary folds. The medullarygroove 

 is developed anteriorly before any 

 trace of it is visible behind. In a 

 general way the closure of the groove 

 takes place as in Birds, but the an- 

 terior part of the body is very early 

 folded off, sinks into the yolk, and 

 becomes covered over by the amnion 

 as by a hood (figs. 127 and 129). All 

 this takes place before the closure of 

 the medullary canal ; and the changes' 

 of this part are quite concealed from 

 view. 



The closure of the medullary canal 

 commences in the neck, and extends 

 forwards and backwards ; and the whole region of the brain 

 becomes closed in, while the groove is still largely open behind. 



The later stages in the development of the Lacertilian 

 embryo do not require a detailed description, as they present 

 the closest analogy with those already described for Aves. 

 The embryo soon turns on to its left side ; and then, becoming 

 continuously folded off from the yolk, passes through the series 

 of changes of form with which the reader is already familiar. 

 An advanced embryo is represented in fig. 130. The early 

 development and great length of the tail, which is spirally 

 coiled on the ventral surface, is a special feature to which the 

 attention of the reader may be called. 



Embryonic Membranes and Yolk-Sack. 



The early development of the cephalic portion of the amnion 

 has already been alluded to. The first traces of it become 

 apparent while the medullary groove is still extremely shallow. 

 The medullary plate in the region of the head forms an axial 

 strip of a thickish plate of epiblast. The edge of this plate 



