2o6 



EMBRYOLOGY 



transform the groove into a tube, beginning at the head end, while 

 the posterior arms of the medullary folds remain asunder, and take 

 the front end of the primitive streak between them. In the mean- 

 time the cleavage of the lateral or more marginal portions of the 



mesoblast into an outer and an inner 

 layer has taken place, the cleft being 

 the future pleuro-peritoneal cavity (r/. 

 This cleavage does not ex- 



p. 203). 



tend throughout 



the 



MC 



T.F. 



whole of the 

 mesoblast, but stops at some little 

 distance to the right and left from 

 the medullary groove and the noto- 

 chord. These uncleft mesoblastic por- 

 tions are called the A^ertebral plates, 

 in opposition to the lateral plates or 

 split portions of the mesoblast. At 

 first the right and left vertebi'al plates 

 remain unbroken along their length, 

 but soon transverse constrictions ap- 

 pear in them, and cut them up into 

 a series of cubical masses. These 

 cubes are called the mesoblastic som- 

 ites or protovertebrse ; they are the 

 basis out of which the voluntarj^ mus- 

 cles of the trunk and limbs, and the 

 bodies of the vertebrie are formed. 

 The first pair of somites rises in level 

 of the anterior end of the primitive 

 sti"eak, the next pairs are added on 

 between the first and the streak, so 

 that the latter seems continually car- 



iNcuBATioN, FROM ABOVE. Magnified j^ed back, the lengthenins; of the em- 

 fifteen times. (After Balfour.) ° ^ 



V.B. MB, HB, Vesicles of the fore-, 

 mid-, and hind -brain ; O.V , Optic 

 vesicle; Au.P, Auditory Pit; A. P. 

 Area pellucida ; Fv. V, Protovertebrae 

 (mesoblastic somites) ; MC, Medullary 

 canal in region of sacral rhanboid COrrespOuds tO the future head, and 



sinus: pr.s. Rest of the primitive ^]^q j.gg^ ^o the ueck, bodv, and tail. 



streak; r.F. Tail-fold of the amnion. ta • n i i ,i 



Durins? ail these changes the area 

 opaca has been spreading over the surface of the yolk, so far that 

 the whole blastoderm at the end of the first day of incubation, has 

 attained the size of a sixpence. Vessels appear in the area opaca 

 in the immediate neighbourhood of the embryo. 



Changes during the 2nd day. — During the first half of this the 

 medullary folds are closing rapidly ; the groove is converted into 

 the neural canal, closed in front, but still open behind. The portion 



Pr.s. 



Embryo of the Common Fowl, 

 from thirty to thibty-six hours of 



bryo always taking place betAveen the 

 front end of the streak and the last 

 somite. All that j^art of the embryo 

 which is in front of the first somite 



