EAELY DBVKLOPMBNT OF LAOERTA MUEALIS. 3 



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 {b p), 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- 

 topore, 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 Osborne, " On the Early Development of the Common Newt," 

 ' Quart. Journ. Mic. Sci.,' vol. xix. 



