THE FORMATION OP THE LAYERS IN AMPHIOXUS. 329 
are represented by the great solid sheets of mesoderm cut 
out from the sides of the archeiiteric roof termed the “ meso- 
dermic bands/’ requires no special demonstration. 
To sum up^ the differences between the development of 
Arnphioxus and the development of the higher Verte- 
brata can be explained on the simple assumption that there 
has been a progressive increase in food-yolk, and that this yolk 
for the most part has been stored in the ventral wall of the 
archenteron, which has been thereby rendered relatively inert. 
Tliis has led to a modification of the process of invagination, 
which retains its primitive features in connection with the 
dorsal lip of the blastopore, but ventrally is changed to a 
process of slipping over or epibole. At the same time the 
processes of folding which give rise to the coelom in Amphi- 
oxus become modified so as to give place to the outgrowing 
of solid masses of cells. 
Turning now to Professor Hubrecht’s account (18) of the 
ontogenetic processes in Vertebrata, we find that he entirely 
reverses the method which I have followed. Instead of ex- 
plaining the more complex development of the higdier forms 
as a modification of that of the simpler forms, he takes the 
development of mammals as a starting-point, and then pro- 
ceeds to read into the development of the lower forms what 
he, finds there. Since in the mammalian egg the cells destined 
to form notochord and mesoderm arise by invagination, there- 
fore they are ectoderm and for them the name “protochordal 
wedge” is given. It does not, however, escape Hubrecht that 
these invaginated cells come into continuity in fi-ont with 
cells which he regards as true endoderm, and therefore in 
front of the protochordal plate there is an endodermic proto- 
chordal plate, and the notochord, if I understand him ai’ight, 
arises from both, and is therefore a compound strnctui’e. 
Further, Hubrecht is a convinced believer in the formation 
of the neural plate by the gradual closing of a long slit-like 
blastopore. This process he dignifies with the name “noto- 
genesis.” To read a complex process like this into the develop- 
ment of Arnphioxus appears to me a sheer impossibility. 
