280 
E. W. MAC BK IDE. 
description of tlie early stnges of the development of this 
animal, based on hundreds of series of sections througdi well- 
preserved material (26). The conclusions arrived at in that 
paper, however, have been combated by several workers 
since, and as the figures illustrating it were hastily executed 
and somewhat schematic, owing to my impending departure 
from England to take up work at McGill, some authors have 
taken for granted that the material was badly preserved and 
that my observations were therefore of little value. Such a 
conclusion is entirely unwarranted, as the figures illustrating 
the present paper will, I hope, convince every unprejudiced 
observer. I have endeavoured, so far as my powers of draughts- 
manship extend, to depict the cellular structure in minute 
detail, and I find, on going over the subject ag-ain after eleven 
years’ interval, that I must re-affirm all the statements I made 
in 1898 with a few trifling exceptions in minor points. 
Part I. 
The Formation of Layers in Amfhioxus. 
Pefoi'e, however, proceeding to the description of the 
development of Am phioxus, it is perhaps desirable to give a 
brief review of former work on the subject so that the ques- 
tions at issue may be clearly grasped. Kowalevsky was the 
first who published a paper (21) on the development of 
Amphioxus, which he followed up some years later by a 
second paper on the same subject (22). In his first paper he 
describes the egg as dividing into blastomeres of nearly 
equal size, forming a hollow blastula. This blastula, according- 
to him, is converted into a gastrula by a flattening of one 
pole, followed by an invagination of the flattened area pro- 
ceeding equally at every point of its circumference, so that 
the blastopore is left as a symmetrically situated posterior 
pore. Ill his second paper Kowalevsky describes the forma- 
tion of “ folds ” of the inner layer (hypoblast) situated 
dorso-laterally. From the context it is clear that by “folds” 
he means pouch-like outgrowths. Ke makes the significant 
