100 



COMPARATIVE PHYSIOLOGY. 



m.bT 



au.p. 



transverse division of the columns of mesoblast that formed the 

 vertebral plates. 



Before the permanent vertebrae are formed, a reunion of the 

 original protovertebrse takes place as one cartilaginous pillar, 



followed by a new segmen- 

 f:-^- tation midway between the 



original divisions. 



It is thus seen that a 

 large number of structures 

 either appear or are clearly 

 outlined during the first 

 day of incubation : the 

 primitive streak, primitive 

 groove, medullary plates 

 and groove, the neural ca- 

 nal, the head-fold, the 

 cleavage of the mesoblast, 

 the protovertebrae, with 

 traces of the amnion and 

 area opaca. 



During the second day 

 nearly all the remaining 

 important structures of the 

 chick are marked out, while 

 those that arose during the 

 first day have progressed. 

 Thus, the medullary folds 

 close ; there is an increase 

 in the number of protover- 

 tebrae ; the formation of a 

 tubular heart and the great 

 blood-vessels ; the appear- 

 ance of the Wolffian duct ; 

 the progress of the head re- 

 gion ; the appearance of the 

 three cerebral vesicles at the anterior extremity of the neural 

 canal ; the subdivision of the first cerebral vesicle into the optic 

 vesicles and the beginnings of the cerebrum ; the auditory pit 

 arising in the third cerebral vesicle (hind-brain) ; cranial flex- 

 ure commences ; both head and tail folds become more dis- 

 tinct ; the heart is not only formed, but its curvature becomes 

 more marked and rudiments of auricles arise ; while outside 



Pig. 103.— Embryo of chick, between thirty 

 and thirty-six honrs, viewed from above 

 as an opaque object (Foster and Balfour). 

 /. 6, forebrain; m. 6, midbrain; h. b, hind- 

 brain; op. V, optic vesicle; au.p, auditory 

 pit ; o.f, vitelline vein ; p. v, mesoblastic 

 somite; m./, line of function of medulla- 

 ry folds abovejmedullary canal ; s. r, sinus 

 rhomboidalis; t, tail-fold; p. r, remains of 

 primitive groove ; a.p, area pellncida. 



