DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 7 



Stage I 

 Figures 2-2/ (Plates I, II) 



The youngest embryo that we have for description is shown in 

 figures 2 and 2a. Of figure 2 Clarke says : 



"The limiting line between the opaque and 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 blastopore is already formed near the posterior end of the shield. 



"A ventral view of another embryo of the same age (fig. 20), 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 begin- 

 ning of a curved depression at the anterior end of the shield, the 

 first formation of the head-fold." 



Transverse sections of this stage are shown in figures 2b— 2f. 



Figure 2b, through the anterior region of the blastoderm, shows a 

 sharply defined ectoderm (ec) which is composed of three or four 

 layers of cells in the median region, while it gradually thins out 

 laterally. Closely underlying this ectoderm is a thin sheet of irreg- 

 ular cells, the entoderm (en). 



Figure 2c is about one-fifth of the length of the blastoderm pos- 

 terior to the preceding and represents approximately the same condi- 

 tions, except that there is an irregular thickening of the entoderm in 

 the median region (en). This thickening apparently marks the an- 

 terior limit of the mesoderm, to be discussed shortly. 



Figure 2d represents the condition of the blastoderm throughout 

 about one-third of its length, posterior to the preceding section. The 

 somewhat regular folds in the ectoderm (ec) are probably not the 

 medullary folds, but are such artificial folds as might easily be pro- 

 duced in handling the delicate blastoderm. The thickening of the 

 entoderm, noticed in the preceding figure, is here more sharply de- 

 fined, and as we pass toward the blastopore becomes separated some- 

 what from the entoderm proper as a middle layer or mesoderm (fig. 

 2e, mes). It would thus seem, from a study of these sections, that 

 most of the mesoderm is derived from the entoderm. In fact, all of 

 the mesoderm in front of the blastopore seems to have this origin, 

 for it is not until the anterior edge of the blastopore is reached that 

 there is any connection between the ectoderm and entoderm (fig. 2e). 



Figure 2e is a section through the region just mentioned, where, 

 medially, the ectoderm, mesoderm, and entoderm form a continuous 



