DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 197 



Our first stage is taken shortly before the tadpole works its way- 

 out of the gelatinous mass of the spawn and becomes free. At this 

 stage our tadpoles have an average length of 4| mm. 



1. External Characters. 



The head is large and prominent, and separated by a very distinct 

 neck constriction from the body. The body is greatly distended 

 ventrally by the yolk mass. The tail is well formed, but short, not 

 exceeding a fourth of the total length of the animal. 



The sucker is well-developed, and forms a prominent object on the 

 ventral surface of the head. The olfactory organs form a pair of 

 shallow circular depressions, with slightly raised margins, at the 

 anterior end of the head. The eyes are already present, but are only 

 visible on the surface as a pair of slightly raised longitudinal ridges, 

 immediately behind the olfactory pits, and continuous with their 

 raised margins. Below the olfactory pits, in the median line, is the 

 stomatodseum, which as yet is a small and very shallow depression. 

 Two pairs of external gills are present as backwardly directed pro- 

 cesses from the first and second branchial arches ; they are some- 

 what conical in shape, with rounded or very slightly notched hinder 

 borders. The gill of the first arch overlaps that of the second, and 

 is placed rather more ventrally than the latter. 



In front of the first gill the hyoid arch may be recognised as a 

 slight vertical ridge, and behind the second gill the third branchial 

 arch is present, though inconspicuous, owing partly to its lying 

 in the constriction between head and body, and partly to its being 

 overlapped by the second gill. 



The myotomes are clearly visible along the whole length of the 

 body and tail ; they are most distinct a short distance behind the 

 head, and are > shaped, with the angles directed forwards as in 

 the adult Amphioxus. 



2. Internal Anatomy. 



The pharynx is of considerable size, being both wide transversely 

 and deep dorso-ventrally (cf. Plate XIV., Fig. 5). In horizontal section 

 it is somewhat diamond-shaped, its anterior angle being prolonged 

 forwards beneath the infundibulum as a hollow, laterally compressed 



