6o 



GENERAL GROWTH OF THE EMBRYO. 



posterior pair, which is the later developed and less conspicuous 

 of the two, is situated 

 some little distance be- 

 hind the stalk. 



The cranial flexure 





ol 



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 

 (Jib) may be noticed in 

 the figure behind the 

 mid-brain. The audi- 

 tory sack (an. 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 ; aii.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 ; an. V. auditory vesicle ; m. mesoblast at base 

 of brain ; ch. notochorcl ; hi. heart ; Vc. visceral 

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

 in the head. 



