54 EDWARD MCCRADY, JR. 



Heuser of the Carnegie Institution. His material, which is 

 very complete, is that which was collected by himself and 

 Doctor Hartman in 1918. They have kindly allowed me to 

 take the figures for my stages 15 to 20 from some of their 

 beautiful photographs. As Doctor Heuser 's account of the 

 next four stages will be much more detailed than I am able 

 to give, I shall content myself here with listing the distinctive 

 features which I have chosen as criteria for defining them. 



Stage 17 



The mesodermal crescent. Hartman 's litters 346 and 346' 

 show the changes which occur between the middle and the 

 end of the seventh day (interval of 9f hours). The eggs of 

 346 represent late stage 14, and those of 346' are types for 

 stage 17. The latter are 2.0 mm. blastocysts characterized by 

 a strongly developed primitive streak from the posterior 

 portion of which mesoderm streams laterally to form two 

 symmetrical wings the mesodermal crescent. 



The remaining three neurula stages (i.e., stages with no 

 sign of organogenesis except the medullary plate) belong 

 to the eighth day, and are described in the next chapter. 



VIII. THE EIGHTH DAY 



Stages 18 to 23. Hensen's node and the primitive groove. 

 Elongation of the medullary plate. The mesodermal rim. 

 The notochord and the medullary groove. The first somites 

 and the parietal mesoderm. The coelomic cavities and the 

 subcephalic fold. 



Stage 18 



Hensen's node and the primitive groove. In the early 

 part of the eighth day when the vesicle has attained a 

 diameter of 2.2 mm. a distinct thickening of the external 

 layer appears at the anterior end of the primitive streak. 

 This is Hensen's node. At this point ectoderm and endo- 

 derm seem to fuse, and mesoderm may be seen proliferating 



