DEVELOPMENT OF THE PRIMITIVE SEGMENTS. 



163 



In this chapter we shall take into consideration the formation of 

 the, primitive segments first in the eggs of Amphioxus and the 

 Amphibians, and then in those of Fishes, Birds, and Mammals. 



In Amphioxus the formation of the primitive segments is more 

 nearly simultaneous with the development of the middle germ- 

 layer than in the remaining Vertebrates. As soon as the two 

 ccelomic sacs begin to grow out from the coelenteron at the front end 

 of the embryo, there begins a division of them into two rows of 

 small sacs lying one behind the other (fig. 103 A, B, us), and this 

 division proceeds from in front backwards. Here again we have 

 to do with a process of folding, which 

 repeats itself many times in the same 

 manner. 



The wall of the groove-like coelomic 

 evagination, composed of cylindrical 

 cells, becomes, at a little distance from 

 its head-end, folded transversely to the 

 longitudinal axis of the embryo ; this 

 fold grows from above and from the 

 side downwards into the body-cavity; 

 in the same manner a second trans- 

 verse fold is soon formed on either 

 side of the body at a little distance 

 behind the first ; behind the second 

 a third, a fourth, and so on, at the 

 same rate as that at which the em- 

 bryonal body elongates and the fun- 

 dament of the middle germ -layer 



increases by the progress of the evagination toward the blasto- 

 pore. 



In the embryo represented in fig. 103 five sacs may be counted on 

 either side of the body. The evagination is taking place at the 

 region marked mk ; it advances still farther toward the blastopore 

 and gives rise to a considerable series of primitive segments, the 

 number of which in a larva only twenty-four hours old has already 

 increased to about seventeen pairs. The primitive segments exhibit 

 at first an opening, by means of which their cavities (usK) are in 

 communication with the intestinal cavity. But these openings soon 

 begin to be closed in succession, by their margins growing toward 

 each other and then coalescing: this takes place in the same sequence 

 as that in which the detachment of the parts takes place, from before 



Fig. 104,-Cross section through 

 middle of the body of an Amphioxus 

 embryo with 11 primitive segments, 

 after HATSCHEK. 



ak, Outer, ik, inner germ-layer ; mk 1 , 

 parietal, mk 1 , visceral lamella of 

 the middle germ-layer ; us, primi- 

 tive segment ; n, neural tube ; ch, 

 chorda; Ih, body-cavity; <Ui. intes- 

 tinal cavity. 



