6o 



GENERAL GROWTH OF THE EMBRYO. 



ol 



posterior pair, which is the later developed and less conspicuous 

 of the two, is situated ^ ^ mr 



some little distance be- 

 hind the stalk. 



The cranial flexure 

 has greatly increased, 

 and the angle between 

 the long axis of the 

 front part of the head 

 and of the body is less 

 than a right angle. The 

 conspicuous mid-brain 

 (29 A, mb) forms the 

 anterior termination of 

 the long axis of the 

 body. The thin roof 

 of the fourth ventricle 

 (hb} may be noticed in 

 the figure behind the 

 mid-brain. The audi- 

 tory sack (au. V} is 

 nearly closed, and its 

 opening is not shewn 

 in the figure. In the 

 eye (op} the lens is 

 completely formed. 

 The olfactory pit (ol} 

 is seen a little in front 

 of the eye. 



Owing to the opa- 

 city of the embryo, the 

 muscle-plates are only indistinctly indicated in fig. 28 F, and no 

 other features of the mesoblast are to be seen. 



The mouth is now a deep pit, the hind borders of which are 

 almost completely formed by a thickening in front of the first 

 branchial or visceral cleft, which may be called the first branch- 

 ial arch or mandibular arch. 



Four branchial clefts are now visible, all of which are open 

 to the exterior, but in the embryo, viewed as a transparent 



FlG. 29. VIEWS OF THE HEAD OF ELASMO- 

 BRANCH EMBRYOS AT TWO STAGES AS TRANS- 

 PARENT OBJECTS. 



A. Pristiurus embryo of the same stage as fig. 

 28 F. 



B. Somewhat older Scyllium embryo. 



///. third nerve ; V. fifth nerve ; VII. seventh 

 nerve; au.n. auditory nerve; gl. glossopharyngeal 

 nerve ; Vg. vagus nerve ; fb. fore-brain ; pn. pineal 

 gland ; mb. mid-brain ; hb. hind-brain ; iv.v. fourth 

 ventricle ; cb. cerebellum ; ol. olfactory pit ; op. 

 eye ; au. V. auditory vesicle ; m. mesoblast at base 

 of brain ; ch. notochord ; ht. heart ; Vc. visceral 

 clefts ; eg. external gills ; pp. sections of body cavity 

 in the head. 



