136 W. F. K. WELDOX. 



tinguished from the anterior by the presence of the primitive 

 streak. 



A median longitudinal section through an embryo with a 

 commencing primitive streak is shown in fig. 1. Anteriorly 

 the area pellucida is seen to be formed by an epiblastic layer of 

 irregular columnar cells and a sheet of lower layer cells, the 

 two layers being quite distinct. At a point {hp), however, the 

 position of the future blastopore, these layers are replaced by 

 a mass of closely-packed cells [pr), exhibiting no division 

 into layers, and forming the primitive streak, which may in 

 some cases at least extend backwards as far as the commence- 

 ment of the area opaca. 



The blastopore commences at the anterior end of this streak 

 as a pit, open above, and closed below. This is shown in 

 fig. 2. 



The floor of this pit presently breaks up, and the blastopore 

 assumes its normal condition, forming a communication between 

 the archenteron and the exterior, its anterior wall forming a 

 communication between the epiblast and the lower layer cells 

 (see fig. 3). 



From this time a change in the character of the lower layer 

 cells takes place, beginning from the anterior wall of the blas- 

 topoie, where they pass into the epiblast, and proceeding for- 

 wards. Instead of being large, irregular, full of yolk, as in the 

 previous stages, they become columnar, lose their yolk, arrange 

 themselves in a definite layer several cells deep, and take on 

 the characters of normal hypoblast. A median longitudinal 

 section through an embryo, in which about half the lower layer 

 cells are thus converted, is seen in fig. 4. . 



This process is evidently an invagination comparable to that 

 which takes place in an Elasmobranch. It especially resembles 

 the process described by Scott and Osborne^ in the newt. 



The first traces of mesoblast appear at a stage slightly earlier 

 than that represented in fig. 4. Fig. 5, which shows a portion 

 of a lateral section from the same series as that to which 



• Scott and Osbornp, "On tbe Earlj Development of the Common Newt,' 

 this Journal, vol. xix. 



