THE EOEMATION OF THE LAYEES IN AMPHIOXUS. 315 
individual cells are larger; (2) the up-growth of the ventral 
lip is more marked and takes place at an earlier stage. 
The whole subject has been fully dealt with by that able 
student of cell lineage, Conklin, in his exhaustive description 
of the development of Cynthia (19), where he has accounted 
for every individual cell and shown its lineage. Conklin 
emphatically states that the closing of the blastopore includes 
two stages, in the first of which the dorsal lip grows back- 
wards, and in the second and subsequent stage the ventral 
lip grows upwards and the lateral lips at the same time 
coalesce. Conklin admits that mesoblast and notochord are 
Text-fig. 4. 
4 . 
Diagrams comparing an advanced gastrula of Amphioxns and of 
Cynthia. (According to Conklin.) 
differentiated and their component cells recognisable before 
they are invaginated, but he utterly declines to call them 
ectoderm, “because they are large and full of yolk, and 
resemble endoderm.” 
When we leave the development of the Ascidian larva, 
which in its sense-vesicle and well-developed tail stands higher 
in the vertebrate scale than Am phi oxus, the next highest 
stage is represented by the Cyclostoine fishes, the develop- 
ment of which has been studied by many observers. Unfor- 
tunately none of these have been specially interested in the 
minute details of gastrulation, but the figures they give of 
both segmentation and gastrulation of the egg are extra- 
