No. 2.] EMBRYOLOGY OF THE LIZARD. 325 



blast {Mp) is also distinctly defined, with its usual shape dor- 

 sally but uniting ventrally below the intestine. Proceeding 

 farther forward, the caudal intestine invariably becomes a solid 

 cord, which gradually disappears. The condition of the tail at 

 this point is shown in Fig. 56, which represents a cross-section 

 just behind the middle length of the tail. From these facts I 

 conclude that the caudal intestine continues to grow in the 

 neurenteric region, even after its anterior part behind the anus 

 has atrophied. The atrophy occurs from before backward, and 

 for a time the proximal end seems to atrophy about as fast as 

 the distal end grows. 



4. The Mesoblast and Primitive Kidney. 



The origin of the mesoblast has been described by the authors 

 referred to in the introduction. I will only refer to a few points 

 in its development. The origin of the head-cavities, which are 

 homologous with the body-cavity, has already been described. 

 The segmentation of the mesoblast into somites occurs from be- 

 fore backward, — the first somite appearing at just the distance 

 behind the ear that would equal the space occupied by one 

 somite. About the time of the segmentation of the mesoblast 

 the dorsal part forming the somites becomes separated from 

 the ventral part, which incloses the permanent body-cavity. On 

 each side the walls of the body-cavity — the somatopleure and 

 splanchnopleure — meet dorsally at an acute angle. The apex 

 of this angle becomes divided off by a longitudinal constriction, 

 and appears at first to form a continuous rod of cells more or 

 less fused dorsally and ventrally with the adjacent mesoblast. 

 This rod later becomes segmented. It is the ** intermediate cell- 

 mass," or rudiment of the Wolffian bodies, and has been de- 

 scribed by Weldon in Lacerta muralis. In an embryo with four 

 somites the parts of the mesoblast are crowded on each other, 

 and are not very distinctly defined. In the posterior region of 

 an older embryo, owing to the greater size of the body, the parts 

 referred to are separated from each other and distinctly marked. 

 In an embryo with nine somites the unsegmented mesoblast, 

 slightly anterior to the region of the neurenteric canal, has not 

 divided into dorsal and ventral parts. A little farther forward 

 this division is taking place, and the ** intermediate cell-mass " 

 appears as pictured by Weldon (Figs. 15 and 17 of Weldon' s 



