EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES, 209 
Owing to its mode of formation the wall of this secondary 
vesicle is composed of two layers which are only separated 
by a narrow space, the remnant of the segmentation 
cavity. 
Two of the stages in the formation of the secondary vesicle 
by this process of involution are shown in Plate X, fig. A 11, 
and A ur. In the second of these the general growth has 
been very considerable, rendering the whole animal much 
larger than before. The cavity of this vesicle, A 111, is that 
of the commencing alimentary canal whose final form is due 
to changes of shape undergone by this primitive cavity. The 
inner wall of the vesicle becomes converted into the wall of 
the alimentary canal or hypoblast, and also into part or the 
whole of the mesoblast. 
During the involution the cells which are being involuted 
undergo a change of form, and before the completion of the 
process have acquired a completely different character to 
the cells forming the external wall of the secondary vesicle 
or epiblast. This change of character in the cells is already 
well marked in fig. a 11. It is of great importance, since we 
shall find that some of the departures from this simple mode 
of development, which characterise other vertebrates, are in 
part due to the distinction between the hypoblast and epi- 
blast cells appearing during segmentation, and not subse- 
quently as in Amphioxus during the involution of the 
hypoblast. 
Kowalevsky (‘ Entwickelungeschichte des Amphioxus’) 
originally believed that the narrow mouth of the vesicle (ac- 
cording to Mr. Lankester’s terminology blastopore) became 
the anus of the adult. He has since, and certainly correctly, 
given up this view. ‘The opening of the involution becomes 
closed up and the adult anus is no doubt formed as in all 
other vertebrates by a pushing in from the exterior, though 
it probably corresponds in position very closely with the 
point of closing up of the original involution. 
The mode of formation of the mesoblast is not certainly 
known in Amphioxus; we shall find, however, that for all 
other vertebrates it arises from the cells which are homo- 
logous with the involuted cells of this animal. 
Since food material is a term which will be very often 
employed, it will be well to explain exactly the sense in 
which it will be used. It will be used only with reference 
to those passive highly refractive particles which are found 
embedded in most ova. 
In some eggs, of which the hen’s egg may be taken as a 
familiar example, the yolk spherules or food material form the 
