FORMATION AND FATE OF THE PRIMITIVE STREAK. 93 



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 situation 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 2, which are drawings of sections of the 

 advancing anterior extremity of the pigmented 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 pigmented 

 area is first formed, and as it extends forwards and ventrally, a slit- 

 like space appears in the middle of its posterior portion. This space 

 first limits the dorsal lip of the blastopore, and then extends forwards 

 and ventrally, following 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 blastopore ; their 

 union ventrally gives rise to the ventral lip, and at the same time 

 defines the posterior limit of the ventral wall of the archenteron (fig 3). 



A diagrammatic representation of a sagittal mesial section of the 

 ovum at the period of completion of the blastoporic lip is given in 

 fig. 4, from which it will be readily seen that, at this period, the 

 ventral wall of the archenteron extends from the point a to the point x. 

 In a portion of its extent the floor of the slit-like cavity lies parallel 

 to the superficial surface of the ovum, but posteriorly it is projected 

 outwards into a deficiency in the external wall (the blastopore), 

 forming in this region the yolk-plug or bouchon d'Ecker. At the same 

 stage the dorsal wall of the archenteron is not co-extensive with the 

 ventral. It extends only from the point a to the point b, which marks 

 the dorsal lip of the blastopore. From the dorsal to the ventral lip of 

 the blastopore the wall of the archenteron is deficient. It will be 

 shown afterwards how this portion of the wall, which we shall call the 

 posterior wall of the archenteron, is completed. 



At present we more particularly desire to call attention to the fact 



