t2 DEVELOPMENT OF THE TADPOLE. 



We must now retrace our steps somewhat, in order to study 

 the progress of those animals which were kept under what may be 

 termed normal conditions. On March nth some of them com- 

 menced to fall out of their investing capsules, and formed small 

 masses at the bottom of the jar ; and next day these were able to 

 move their tails slightly, so as to wriggle a little higher up the 

 glass. From March 12th to the 21st the weather was very cold, 

 and development was arrested to a very considerable extent ; but 

 by the latter date many had external gills well developed and 

 could swim easily, but preferred to remain close to the glass. 

 Whether attached or not by their claspers is very difficult to state, 

 as they could be removed with the slightest touch of a small 

 pencil-brush, so that the attachment, if existing, was of the very 

 slightest character. This, to me, seemed the more surprising, as 

 the microscopic preparations showed that the claspers at this 

 period were apparently powerful muscular appliances, capable of 

 producing a partial vacuum if applied to a smooth surface. I 

 therefore anticipated much greater difficulty than was really expe- 

 rienced in their removal. 



The more prominent organs which had developed by this time, 

 although not yet arrived to perfection, were : — (i) Brain, with 

 cerebral vesicles, medulla oblongata, and medulla spinalis; (2) 

 Notochord, terminating a little behind the pituitary vesicle ; (3) 

 Branchial, hyoid, and mandibular arches ; (4) Pericardium, with 

 enclosed heart; (5) Claspers; (6) Mouth-cavity; (7) Pro-renal 

 ducts ; (8) Somites ; (9) Special-sense Capsules, as the auditory 

 and optic and olfactory capsules, from which we shall subsequently 

 find the ear, eye, and nasal organs to proceed. 



[to be CONTINUED.] 



EXPLANATION OF PLATES VII. AND YIII 



Plate VII. 

 Fig. 1. — External appearance of the Tadpole, March 7, showing for- 

 mation of g. , gill arches. 

 ,, 2, — Vertical longitudinal section of same, showing bnc, brain 

 with neural cavity; g., gill arclies ; no., notochord; fn., 

 frontal nasal process ; 5., somites ; >j., yelk mass. 



