702 



must conclude therefore from the preceding experiments that the 

 material for the lateral and posterior lips of the blastopore appears 

 as a riDg of cells around the black-white line. The closure of the 

 blastopore is brought about by the growth towards the middle line of 

 the material of the ring. The fusion of the two sides of the ring 

 progresses from before backwards, so that new material is constantly 

 brought up to the dorsal lip of the blastopore from the sides. 



Formation of the Archenteron. 



A study of serial sections of both normal and abnormal embryos, 

 combined with the foregoing data, has thrown much light upon the 

 process by which the walls of the archenteron are formed in the frog. 



Before the appearance of the crescent of the blastopore, sections 

 through the embryo show a thickened zone of cells lying just outside 

 of the black-white line, and below the sides of the segmentation ca- 

 vity. These cells are smaller than the large yolk bearing cells, and 

 have smaller yolk granules; they have pigment around their nuclei, 

 and are larger than the outer ectoderm cells from which they are 

 separated by no sharp line. These cells, as later development shows, 

 are largely ectodermal but the innermost are mesodermal. When 

 the crescentic dorsal lip of the blastopore appears, it is formed just 

 inside of the black-white line, i. e. below this thickened ring of cells. 

 As the crescent extends laterally and finally forms a circle, it lies 

 along a line just inside of the white. Serial sections through both normal 

 and abnormal embryos show that the cells along the blastopore crescent pull 

 in from the surface, leaving only their small pigmented ends for a time 

 exposed. This process continues, the elongated, cells pull in beneath the 

 surface and a narrow space is formed between the yolk cells. The 

 yolk cells, of the anterior and outer wall, are somewhat smaller cells 

 than those forming the inner surface of the cavity. This is because the 

 invagination took place along the line where the larger cells pass into 

 the smaller cells. As the crescentic blastopore extends laterally the 

 same process of invagination (or pulling in) of surface cells takes place. 



Meanwhile the lateral lips of the blastopore have begun to roll 

 in from the sides towards the median line, so that the dorsal lip ap- 

 pears to move backwards over the surface of the egg. As the lateral 

 lips roll in, the cavity formed by invagination (archenteron) gets deeper. 

 When it is remembered that the lateral lips of the blastopore reach 

 the median line from before back, so that the dorsal lip advances by 

 the addition of new cells to its posterior end, we can understand that 

 the dorsal wall of the archenteron will increase in length by the ad- 



