108 



COMPARATIVE PHYSIOLOGY. 



An examination of the figures and subjoined descriptions 

 must suffice to convey a general notion of the subsequent prog- 



yji. 



MP: 



Fig. 114.— Embryo at end of fourth day, seen as a transparent object (Foster and 

 Balfour). CH, cerebral hemisphere; F. B, fore-brain, or vesicle of third ventricle 

 (thalamencephalon), with pineal eland (Pn) projecting; M. B, mid-brain; C.b, 

 cerebellum; IV. V, fourth ventricle; L, lens; chs, choroid slit; Cen. V, auditory 

 vesicle; sm, superior maxillary process; IP, 2F, etc., first, second, etc., visceral 

 folds ; K fifth nerve; VII, seventh nerve; G, Ph, glossopharyngeal nerve; Pg, 

 pneumogastric. The distribution of these nerves is also indicated ; ch, noto- 

 ohord; Ht, heart; MP, muscle-plates; W, wing; H. L, hind-limb. The amnion 

 has been removed. At, allantois protruding from cut end of somatic stalk SS. 



ress of the embryo. Special points will be considered, either in 

 a separate chapter now, or deferred for treatment in the body 

 of the work from time to time, as they seem to throw light 

 upon the subjects under discussion. 



DEVEIfOMENT OF THE VASCULAR SYSTEM IN VER- 

 TEBRATES. 



This subject has been incidentally considered, but it is of 

 such importance morphological, physiological, and pathological, 

 as to deserve special treatment. 



In the earliest stages of the circulation of a vertebrate the 

 arterial system is made up of a pair of arteries derived from the 

 single bulbus arteriosus of the heart, which, after passing f oi^ 



