1 92 CLARKE. [Vol. V. 



nous white lying between the germ and the shell. And as 

 growth and development proceed, it becomes essential to estab- 

 lish closer connection and freer communication with the exterior 

 for respiration, excretion, etc. ; and this is secured by bringing 

 the germ to the side, where there is comparatively little between 

 it and the air without. 



It is my purpose in the present paper to furnish a general 

 account or outline of the forming of the alligator, as seen in 

 the external features, as completely as my material will allow. 

 I have been led to do this by reason of (i) the entire lack of 

 any embryological knowledge of the alligator group, 1 and (2) on 

 account of there still being something to be desired in the way 

 of a set of general figures illustrating the development of a 

 reptile. Later I hope to publish the results of studies of the 

 internal development. The most complete series hitherto is 

 that of Lacerta agilis> given by Strahl in his well-known papers 

 from 1 88 1 to 1884. 



The series that I can give is at several points incomplete ; 

 but as the eggs are each year more difficult to get, and as it is 

 now nearly three years since I first went to Florida for them, 

 it is perhaps wiser to publish than to wait longer. 



The Formation of the Embryo. 



The youngest embryo that I received is shown in Fig. 12, 

 PL X, as seen from the dorsal side. The limiting line between 

 the opaque and the pellucid areas is clearly marked, and within 

 the latter is a shield-shaped area connected by the narrower 

 region of the primitive streak with the area opaca. The blasto- 

 pore is already formed near the posterior end of the shield. 



A ventral view of another embryo of the same age, seen from 

 the ventral side, shows that the blastopore extends quite through 

 the blastoderm, in an oblique direction downwards and for- 

 wards, from the dorsal to the ventral side. The thickened area 

 of the primitive streak is here very prominent. There is, too, 

 the beginning of a curved depression at the anterior end of the 

 shield, the first formation of the head-fold. The head-fold rap- 

 idly increases in depth and prominence, as shown in Fig. 14, 



1 One must except the posthumous paper of Rathke, 1 866, which deals only with 

 a few of the later stages. 



