ELA SMOBBA NGHII. 



49 



able during the last stage, is now a more conspicuous organ. There 

 are three visceral clefts, none of which are as yet open to the exterior. 



Figure 28 F represents a considerably older embryo viewed as an 

 opaque object, and fig. 29 A is a view of the head as a transparent 

 object. The stalk connecting it with the yolk is now, comparatively 

 speaking, quite narrow, and is of sufficient length to permit the 

 embryo to execute considerable movements. 



The tail has grown immensely, but is still dilated terminally. 

 The terminal dilatation is mainly due to the alimentary vesicle (fig. 28* 

 alv)^ but the postanal section of the alimentary tract in front of this 

 is now a solid cord of cells. Both the alimentary vesicle and this cord 

 very soon disappear. Their relations are shewn in section in fig. 28*. 



The two pairs of limbs 



have appeared as differen- vqr gr_ auv 



tiations of a continuous but 

 not very conspicuous epi- 

 blastic thickening, which 

 is probably the rudiment 

 of a lateral fin. The an- 

 terior pair is situated just 

 at the front end of the 

 umbilical stalk ; and the 

 posterior pair, which is the 

 later developed and less 

 conspicuous of the two, is 

 situated some little dis- 

 tance behind 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, mh) forms the an- 

 terior termination of the 

 long axis of the body. The 

 thin roof of the fourth ven- 

 tricle {Jih) may be noticed 

 in the figure behind the 

 mid-brain. The auditory 

 sack iau.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 [pV) is seen a 

 little in front of the eye. 



B. E. II. 



Fig. 29. Views of the head op Elasmo- 

 branch embryos at two stages as transparent 



OBJECTS. 



A. Pristiurus embryo of the same stage as fig. 

 28 F. 



B. Somewhat older Scyllium embryo. 



III. third laerve; V. fifth nerve; VII. seventh 

 nerve; axi.n. auditory nerve ; gl. glossopharyngeal 

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

 gland; mh. mid-brain; hh. hind-brain; iv.v. fourth 

 ventricle ; ch. cerebellum ; ol. olfactory pit ; oj). 

 eye; an.V. auditory vesicle ; m. mesoblast at bafe 

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

 clefts; eg. external gills; j)p. sections of body- 

 cavity in the head. 



4 



