THE FOEMATION OF THE LAYERS IN AMPHIOXUS. 281 
statement tliat the cavities of the first pair of these folds ” 
remain for a long time widely open to the gut, whilst the 
cavities of the others have very narrow communications with 
the gut and are soon cut off entirely from it. Further, he 
states that whilst the first pair of folds become converted into 
hollow epithelial vesicles, the cavities of the other pairs 
become quite obliterated. He was not able to trace these 
folds into the formation of the body-cavity of the adult, but 
he believed that a genetic connection existed between the 
two sets of organs. 
After an interval of some years Kowalevsky was followed by 
Hatschek, who, in a brilliant paper, gave the account of the 
development of Amphioxus which has been incorporated in 
all the text-books of zoology (15). Hatschek pointed out that 
in the formation of the blastula the blastomeres at one pole 
are larger that at the other, and that it is the area consisting 
of the larger blastomeres which is invaginated when the 
gastrula is formed. He observed further that when gastrula- 
tion is complete the blastopore was situated at the posterior 
end of the dorsal surface. He drew the conclusion that in 
the early stages of gastrulation, when there is a wide-open 
blastopore, this is directed towards the future dorsal surface 
of the animal, and that it becomes reduced in size by the 
meeting of its edges, a process which he supposed to take 
place from the fi'ont backwards. Hatschek never asserted 
that he had seen this union or “concrescence” of the lateral 
edges of the blastopore ; and it is hard to see why, if such 
a process really took place, some trace of it, in the form of a 
seam or raphe, should not be observed. Once the gastrulation 
has been completed the processes of formation of the noto- 
chord and of the mesoderm are begun. These processes, 
according to Hatschek, consist in the production of three 
longitudinal folds of the dorsal wall of the gut, of which the 
median forms the notochord and the lateral the mesoderm. 
These “folds” are genuine longitudinal hollow ridges, not 
“folds” in the sense in which the word is employed by 
Kowalevsky. It is important to note this because many who 
