286 OUTLINES OF CHORDATE DEVELOPMENT 



The general changes in form and the development of the 

 externally visible organs may best be appreciated by studying 

 such a series of embryos as that illustrated by Duval (Atlas 

 d'embryologie, Paris, 1889; Plates VII-X). We may merely 

 call attention to a few of the more striking features. The fore- 

 and hind-limbs appear, at approximately the same time, about 

 the eightieth hour, as bud-like outgrowths from the body wall 

 (Fig. 114). These enlarge rapidly; their segments are distin- 

 guishable at the beginning of the seventh day, and during the 

 eighth day they assume the outlines of the wings and legs. 

 During the eighth day too, the heart is drawn within the body 

 wall, the visceral arches and gill clefts disappear, save as they 

 remain represented in the adult by the jaws (mandibular arch) 

 and the external auditory meatus (hyonjandibular gill cleft), 

 and the abdominal viscera are much enlarged. The following 

 day the facial portion of the head begins to enlarge, the feather 

 papillae become well marked, and the general musculature begins 

 to develop to such an extent as to affect the general contours 

 of the body. 



VII. THE EMBRYONIC MEMBRANES AND 

 APPENDAGES 



We have now to describe the development of certain ex- 

 tremely important structures which are primarily extra-em- 

 bryonic, and have to do with the protection, nutrition, and 

 respiration of the embryo, rather than with the actual forma- 

 tion of proper organs. These structures are the yolk-sac, the 

 amnion, and the allantois; strictly speaking the allantois arises 

 as an embryonic structure, but it soon becomes extra-embryonic 

 in position, and its functional value is derived from its later 

 relation. These three structures are all characteristic of the 

 embryos of the higher Craniates Reptiles, Birds, and Mam- 

 mals, and consequently these classes are often grouped to- 

 gether under the term Amniota. 



1. The Yolk-sac 



We have seen that at the thirtieth hour of incubation the 

 blastoderm has extended over approximately one-fourth the 



