THE EARLY DEVELOPMENT OF AMPHIOXUS. 599 



may term the coelomic grooves) are due to an independent 

 process of folding, originating in the endoderm, is also shown 

 by the fact that the endoderm is no longer in close contact 

 with the ectoderm, a distinct slit-like blastocoele being ob- 

 servable in several places, which certainly would not be the 

 case if the endoderm merely passively followed the foldings of 

 the ectoderm. 



Shortly after the appearance of the coelomic grooves a fresh 

 pair of outgrowths from the alimentary canal make their 

 appearance anteriorly. These, which will be denominated 

 collar cavities, are shown in fig. 13. As will be seen, they 

 are situated slightly nearer the middle line than the coelomic 

 grooves (whicli in the figure are seen external to themj separated 

 by a very narrow fold of the gut wall), and their lumiua are at 

 first excessively narrow (at least in preserved specimens), but 

 they soon enlarge^ and their openings into the cavity of the 

 alimentary canal become clear (fig. 14, c). About the same 

 time the front part of the coelomic grooves becomes constricted 

 off from the alimentary canal, and thus a definite " somite " 

 or myomere is formed (fig. 14, b). Behind, however, the 

 coelomic grooves still open into the alimentary canal, as shown 

 in fig. 14, a. If we compare with such a series of sections 

 through older embryos, such as those figured in fig. 17, 

 d and e, we shall arrive at a clear comprehension as to how 

 the coelomic grooves are converted into a series of somites. 

 We can always find at the hinder end of the embryo appear- 

 ances (like those represented in figs. 14, a, and 14^ c) of the 

 gut wall being folded so as to produce a pair of coelomic 

 grooves, and we can follow the walls of the fold constituting 

 the coelomic groove into the walls of the last somite (compare 

 fig. 17, d). The cavity of no other somite communicates with 

 the gut; it is only the last somite, for the time being, 

 whose cavity is in communication with the gut cavity through 

 the coelomic groove; and hence we see that as the embryo 

 grows in length, and the coelomic groove with it, this latter 

 becomes progressively constricted from the gut and divided 

 into somites at the same time, each new piece which is con- 



