No. 2.] EMBRYOLOGY OF THE AMERICAN ALLIGATOR. 197 



and in none of my embryos do either of them open to the ex- 

 terior. It is so difficult to determine from a surface view 

 whether a cleft is actually open to the exterior, that I have 

 made five series of sections, upon a study of which is partly 

 based the above statement. In all five the first cleft is open, 

 in three the second is open, and in two the third is open. 



The differences in the different specimens are due undoubt- 

 edly to the somewhat different stages of development they are 

 in. A longitudinal, dorso-ventral section (Fig. 42) represents 

 the five clefts ; the anterior three complete, the fourth and fifth 

 rudimentary. There exists then a groove on the inner side of 

 the throat-wall extending from the first to the second visceral 

 cleft, and it disappears as these clefts become complete. A 

 similar groove extends from the second to the third, and from 

 the third to the fourth and fifth. One may see, too, the greatly 

 twisted form of the heart in Fig. 35, and also the nasal pits. 



There is another peculiarity in the pharyngeal clefts. It is 

 the apparent bifurcation of the dorsal end of the first, as shown 

 in Figs. 38 and 39. The stem of the Y and the anterior arm 

 are the true cleft, while the posterior arm is only an external 

 groove, as is demonstrated by sections. The groove and the 

 stem of the Y are about parallel with the remaining clefts, while 

 the anterior arm is decidedly out of line with them. The veins 

 for returning blood from the yolk-sac are seen entering the 

 body immediately posterior to the heart, while the arteries to 

 the yolk-sac leave the body much farther back. The notochord 

 shows plainly throughout most of its length, and several neuro- 

 meres still persist anterior to the ear. The allantois has become 

 very .prominent, projecting from the posterior end of the body 

 cavit}". The rudiment of the fifth nerve has also appeared. 

 The, formation of the nasal pits is illustrated in Fig. 37. The 

 pit is connected by a groove with the cavity of the mouth, the 

 fronto-nasal groove. The limbs are seen developing in Figs. 43 

 and 44, PI. XIII. The shape of the head is by this period 

 very much changed, and the lower jaw is still disproportionately 

 small. The upper jaw is noticeably hooked in Fig. 44, suggest- 

 ing that of a turtle, and looking much like the upper jaw of an 

 embryo of Laccrta muralis shown in Fig. 45. This latter is 

 taken from Balfour's " Comparative Embryology," and was 

 drawn by Professor Haddon. There is nothing peculiarly rep- 



