THE EARLY DEVELOPMENT OF AMPHIOXUS. 595 



egg as being observable in the earlier stages of segmentation, 

 I have been quite unable to find any trace of it in sections, 

 and in this respect confirm Lwoff (10). In agreement with 

 the latter author, I find when the first change ushering in 

 the process of invagination takes place, namely, a flattening 

 of one side of the blastula, that then for the first time a 

 difl'erentiation of the cells composing the embryo into two 

 sorts become observable. Some of them, in fact, become 

 taller and slightly narrower than the rest. Lwofi* lays great 

 emphasis on this phenomenon. He regards the more cylin- 

 drical cells as alone representing the endoderm of Inverte- 

 brates, any further portion of the blastula which may be 

 involved in the process of invagination being denominated as 

 ectoderm. 



It is difficult to find words to adequately characterise the 

 artificiality and arbitrariness of such a view. The only 

 circumstances under which it could be maintained would be if 

 the supposed endoderm were sharply marked off from the 

 ectoderm, and if further there were a pause in the process of 

 gastrulation after the so-called endoderm had been invaginated, 

 but before the invagination of the ectoderm had commenced. 

 A glance at PI. 43, figs. 2 and 3, will show that the taller cells 

 on one side shade imperceptibly into the shorter and rounder, 

 so that it is impossible to say where the one begins and the 

 other ends. On the other side, it is true, there is an abrupt 

 transition at the point marked x. This point I have found by 

 careful comparison with one another of successive stages to 

 correspond to the dorsal (anterior) lip of the blastopore, the 

 very place where Lwoff supposes ectoderm to be invaginated. 

 This spot is easily recognisable in the earlier stages of invagi- 

 nation (PI. 43, figs. 4, 5, and 6), but becomes less recognisable 

 in the later stages. As a tentative explanation of it I may 

 suggest that here an active multiplication of cells takes place, 

 and that those which are added to the invaginated portion of 

 the blastula become laterally compressed and columnar, 

 ■whereas those added to the ectoderm remain stretched by the 

 internal turgidity of the fluid in the blastocoele or segmenta- 



