4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 54 



general features in the development of the alligator, described what 

 he then supposed to be the epiphysis. 



Voelzkow (51), whose paper the writer had not seen at the time 

 of the publication of his above-mentioned paper, quotes Sorensen in 

 the statement that, in the alligator, the paraphysis had been wrongly 

 taken for the epiphysis. In this paper Voelzkow says that the 

 epiphysis is absent in both the Madagascar Crocodile and the Caiman. 

 His figures show conditions very similar to those to be described 

 in the present paper. 



For obvious reasons sagittal sections were chiefly used in studying 

 the development of the paraphysis and hypophysis, though sections 

 cut in other directions were used to some extent. 



The earliest stage in which any sign of the paraphysis is seen 

 is shown in figure i, a sagittal section of an embryo of about 7 mm. 

 length, though the exact size, owing to the marked body flexure, 

 could not be determined. At this stage there is not much difference 

 in the thickness of the walls of the brain, in the different regions, 

 except for the marked thinness of the roof of the hind-brain. A 

 ventral depression, v., of the dorsal wall of the fore-brain is the be- 

 ginning of the velum ; a wide arch, p. a., just anterior to this marks 

 the position of the paraphysis, and may be called the paraphysal 

 arch. Posterior to the velum the roof of the fore-brain is slightly 

 arched to form the beginning of what may be called the post-velar 

 arch, V. a. (Minot), the dorsal sac or zirbelpolster of other writers. 

 An indistinct thickening, p. c, of the dorsal wall marks the posterior 

 limit of this arch, and also the boundary between the fore- and mid- 

 brains ; it is the future posterior commissure. 



Ventral to the brain are seen the anlage of the hypophysis, h., 

 to be described later, and the vacuolated notochord, ch. At this 

 stage of development the brain cavity is very wide, dorso-ventrally, 

 in proportion to its antero-posterior length. 



Figure 2 represents the anterior end of an embryo of about 10 mm. 

 length. It will be noticed that the brain is much longler, in propor- 

 tion to its depth, than in the preceding figure, and that the cranial 

 flexure is more marked. The section passes almost exactly through 

 the median plane of the roof of the brain, but a little to one side of 

 this plane in the region of the hind-brain, so that the notochord is not 

 shown. 



The fore-brain, f. b., is beginning to push forward to form the 

 cerebral hemispheres, c, and a marked fold, the cerebellum, cb., is 

 seen between the mid- and hind-brains. The hypophysis, h., cut 

 slightly to one side of the median plane, has increased in size and 

 complexity ; it will be described later. 



