86 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



with these mesodermal folds of Amphioxus. He has observed that in both asci- 

 dians and Amphioxus these cells are more rounded than other cells of the gastrula. 

 Like Klaatsch, Morgan and Hazen find that around and within the ventral lip of 

 the blastopore, during the early gastrula stages, there are frequently found small 

 rounded cells which contain little yolk. They affirm that the form of these cells 

 is not the result of cell division, as Samassa had assumed, but that they preserve 

 their rounded form even in the resting stage. 



Samassa ( 1S98), however, says that in Amphioxus the origin of the mesoderm 

 has no relation to the blastopore. The fact that the mesoderm has it growth 

 zone at the caudal end of the embryo, in the vicinity of the blastopore, is, he says, 

 a condition which it shares in common with all other organs of the embryo. In 

 the face of the positive evidence adduced by Lwoff, Klaatsch, Morgan and Hazen 

 this negative conclusion of Samassa's seem to me to lose much of its weight. 



It seems probable from these accounts that mesoderm cells are present in 

 the ventral lip of the early gastrula of Amphioxus just as in the ascidians, and 

 that they give rise to the longitudinal mesodermal folds of later stages ; it remains 

 to be seen whether these mesoderm cells may not be traced back to a still earlier 

 stage, comparable with the crescent in the ascidian egg (cf. text (ins. XXVII- 

 XXXII). 



The origin of the mesoderm in amphibians is a much more difficult and com- 

 plicated question and one into which I cannot enter full}' here. It is generally 

 believed, however, that in the frog's egg the cells which are to form the mesoderm 

 are present when the dorsal lip first appears, and even prior to that time. They 

 are the deeper layer of cells of the blastoporic ring and, therefore, surround the 

 egg below the equator. Whether at their first appearance they surround the entire 

 blastopore is not plain, but in later stages this is said to be the case. According to 

 this view the notochord is a mesodermal structm*e differentiated out of the con- 

 tinuous ring of mesoderm surrounding the blastopore. There is here resemblance 

 to the chorda-mesenchyme ring which is present in the ascidians and probably also 

 in Amphioxus, but in the amphibians this ring appears to give rise at once to a 

 sheet of mesoderm and not to mesodemal bands such as are found in Amphioxus 

 and ascidians (text figs. XXXIII-XXXVI1 1 ). 



On the whole it is probable that there is fundamental agreement between 

 Amphioxus and ascidians in the place and manner of mesoderm formation, and 

 though the amphibians differ in some important respects from the other two classes 

 it is possible to interpret their method of mesoderm formation in the same general 

 terms. 



Referring to Raid's (1892) " Theorie des Mesoderms," Samassa (1898), and 

 Garbowski (1898) maintain that there is no peristomal" mesoderm in Amphioxus, 

 but that all the mesoderm is "gastral." If the view here taken is correct, all the 

 mesoderm of this animal is at first peristomal while the gastral mesoderm is later 

 derived from this. This is exactly the conclusion which has been readied by Davidoff 

 (1891), and Castle (1896), with regard to the ascidian, a conclusion which I can 



