FORMATION AND FATE OF THE PRIMITIVE STREAK. 463 



yolk-plug, and when the epiblast formation has arrived from 

 all sides at their margin, the circular blastopore or anus of 

 Rusconi is first definitely established. 



During the period of dififerentiation of the epiblast the 

 archenteron has also been forming. The first indication of 

 this cavity is the appearance of a curved area of pigmentation 

 (fig. 1 a and fig. 2 AR) of semilunar outline amidst the yolk- 

 cells at the posterior pole of the ovum above the equator. The 

 convexity of the pigmented area is directed forwards amidst 

 the yolk-cells, towards the segmentation cavity. The angles 

 are turned ventro-laterally. The centre of the concavity of 

 the semilunar area corresponds to the dorsal lip of the blasto- 

 pore. As the convexity of the area extends forwards towards 

 the segmentation cavity, its lateral angles travel ventrally along 

 the lines of the lateral lip of the blastopore until they meet in 

 the situation of the ventral lip of that opening, which is thus 

 marked out by a line of pigmentation that indicates the situa- 

 tion of the future cavity. 



The pigmented area is produced and extended by the deposit 

 of pigment in the adjacent margins of a double row of yolk- 

 cells in the manner shown in figs. 1 and 3, which are drawings 

 of sections of the advancing anterior extremity of the pig- 

 mented area of a much more developed ovum. The pigment is 

 deposited in the protoplasm of the double row of cells which, 

 eventually, will form the boundary wall of the archenteron, 

 and it also radiates from this area along the adjacent margins 

 of the cells of each row. 



It is to be understood, however, that directly after the pig- 

 mented area is first formed, and as it extends forwards and 

 ventrally, a slit-like space appears in the middle of its pos- 

 terior portion. This space first limits the dorsal lip of the 

 blastopore, and then extends forwards and ventrally, fol- 

 lowing the deposit of pigment, and separating the two rows 

 of marginally pigmented cells from each other. It is the 

 archenteric space. As its lateral angles extend ventrally along 

 the line of pigmentation they define the lateral lip of the blas- 

 topore ; their union ventrally gives rise to the ventral lip, and 



