The Development of the AlHgator 273 



similar to that shown in Figure 11^, which is about 

 the twentieth section posterior to Figure i ij. The 

 break (ec') in the ectoderm is here seen as a com- 

 pact group of cells which at first glance seem to be 

 continuous with a rounded mass of cells below (ps) . 

 Examination under greater magnification, how- 

 ever, shows that the two groups of cells are distinct. 

 As the sections are followed back of this region, the 

 upper mass of cells (ec^) gradually disappears, and 

 after its disappearance the lower mass (ps), which 

 is already continuous with the mesoderm (mes) on 

 either side, becomes continuous with the under 

 side of the ectoderm. The mass of cells (ps) is 

 apparently the primitive streak, though it is dis- 

 tinct from the ectoderm for a considerable distance 

 posterior to the neurenteric canal. Just what 

 may be the meaning of the thickened ridge of 

 ectoderm (ec) it is difficult to determine. 



STAGE IX 

 Figures i2-i2g (Plates XVIII., XIX.) 



The entire length of the embryo proper is 6.5 

 mm. from the extreme posterior end to the region 

 of the midbrain (v^) , which now, on account of the 

 cranial flexure, forms the most anterior part of 

 the body. Besides being slightly longer than the 

 preceding stage, the embryo has increased in thick- 

 ness, especially in the anterior region, where the 

 enlargement of the cerebral cavity is considerable. 



18 



