GO, THE SECOND DAY. [CHAP. 



Behind the Rplanchnopleure fo'd, markinsf the hind lim'ts of the foregut, 

 are seen the two rows of protovertebrap, the dark line between which m. c. 

 indicates the position both of tlie line of junction of the medullary folds and of 

 the notochord. Tlie front end of the notochord is seen at ch. underneath the 

 forebrain ; its hind end is indistinct. Towards the tail the proto vertebrae 

 become indistinct and give place to the vertebral plates v. pi. Still further 

 back, at the commencing tail, all the parts become indistinct, the remains of 

 the primitive groove pv. being as conspicuous as anything else. 



and makes its appearance in the early hours of the second 

 day. Behind it a second and a third bulb, the second and 

 tJdrd cerebral vesicles, are successively formed in a similar 

 manner; but the consideration of these, though they begin 

 to make their appearance soon after the formation of the 

 first cerebral vesicle, may be conveniently reserved to a later 

 period. 



3. The number of proto vertebrse increases rapidly. 

 The one or two pairs which are seen at the end of the first 

 day have by the middle of the second day multiplied to five, 

 or eight, or even more, Figs. 14, 15, p.v, each being formed in 

 the same way as the first. As was mentioned previously, 

 the chief increase takes place from before backwards, the 

 new protovertebrse appearing behind the old ones; but one 

 ])air at least is probably formed in front of that which was 

 the very first to appear. 



In the early part of this day the formation of new proto- 

 vertebrae keeps pace with the closing in of the medullary folds, 

 so that that part of the canal which is already closed in is 

 always fianked by protovertebrae ; but later on the formation 

 of protovertebra3 lags behind, so that for some distance to- 

 wards the hinder extremity the closed medullary canal is 

 unprotected by protovertebrje. Fig. 15. At the extreme end 

 the medullary folds become shallower, diverge from each 

 other, and afterwards meet aarain, thus forming a lozenge- 

 shaped open depression known as the sinus rhomboidalis, 

 Fig. 15, s. r. 



Bphind the sinus rhomboidalis there may generally be seen a small and 

 usually curved remnant of the primitive groove. Eig. i^t P- >"• 



4. In a former chapter it was pointed out (Chap. ii. § 5) 

 that the embryo is virtually formed by a folding or tucking 

 in of the limited portion of the blastoderm, first at the anterior 

 extremity, and afterwards at the posterior extremity and at 



