200 CLARKE. [Vol. V. 



VI. The pharyngeal clefts are five in number, and the first 

 three become open to the exterior. The anterior one appears 

 first as an internal groove lined by thickened epithelium ; this 

 groove also extends backward and enlarges dorso-ventrally into 

 the second cleft, and in like manner, growing backward, gives 

 origin to the third, fourth, and rudimentary fifth. All traces of 

 the groove between the clefts soon disappear. 



The very complete series of perfectly reliable figures of the 

 development of the bird given by Kupffer and Benecke (14) 

 afford a most satisfactory opportunity of comparison with the 

 reptilian type as represented by the alligator. The general 

 appearance of the areas is at first very similar, but there is the 

 absence of a blastopore in the bird ; secondly, the primitive 

 streak and groove are much longer in the bird ; third, there is no 

 secondary fold in the head-fold in the bird, as I have described 

 for the alligator. As the primitive streak decreases in the bird, 

 and the mesoblastic somites begin to form, the embryos of bird 

 and alligator are almost indistinguishable, — the opaque and 

 pellucid areas, the embryonic area, the head and neural folds, 

 the primitive streak, the mesoblastic somites, and the noto- 

 chord are almost identical in both. 



A little later, when the mesoblastic somites have increased to 

 ten or more, the cephalic region of the alligator becomes more 

 opaque than that of the bird, and the entire embryo of the lat- 

 ter is decidedly longer in proportion to the width. The differ- 

 ence in length of the two embryos is just about what it was 

 at the time of the primitive streak, when the latter area was so 

 much longer in the birds. The close resemblance is maintained 

 until the tail of the alligator begins to lengthen and the eyes 

 show their elliptical outline. Even so late as the fifth week of 

 incubation, the snout of the alligator is not much elongated, 

 the head is still at right angles to the spinal axis, and the aspect 

 of the whole head is much more that of a bird than a reptile. 

 The long persistence of the general sauropsidan features is 

 very striking. 



Turning now to the other reptilian groups for comparison, 

 one is disappointed to find that there is not a series of figures 

 which afford a chance for careful or anything like a complete 

 study. The best opportunity in the Lacertilia is offered by the 

 fifteen general figures given by Strahl in his papers on Lacerta 



