286 
E. W. MACBRIDE. 
paper appeared by Morgan and Hazen (28), illustrated by 
beautiful figures. These authors support Lwoff’s statement 
that there is a histological difference between the cells 
forming the dorsal and those forming the ventral wall of the 
archenteron. Their account of the matter is, liowevei’, very 
confusing. In one place they speak of “ yolkless cells 
being inflected at the dorsal lip of the blastopore; in other 
places, however, they represent the difference between the 
roof and floor of the archenteron as merely a difference in the 
intensity with which stain is taken up, and further assert that 
this difference cannot be seen in specimens preserved by 
corrosive sublimate. Their figures certainly show an immense 
difference between the ectoderm cells and those forming 
the roof of the archenteron, whilst the difference between 
the floor and the roof of the archenteron on which they lay 
such stress is only indicated by the colour of the yolk- 
granules. Morgan and Hazen do not accept Lwoti^s view 
that the cells forming the roof of the archenteron ai’e ecto- 
derm, and they point out that mitoses in the dorsal lip of the 
blastopore are no proof of a migration of cells round the lip, 
and that, as matter of fact, mitoses are by no means confined 
to this lip but occur everywhere in both ectoderm and 
endoderm. They assert that in my earlier paper I had 
confused the dorsal and ventral lips of the blastopore, and 
they find a gradual passage from ectoderm to endoderm cells 
at the dorsal lip. After considering all the views which had 
np till then been put forward as to the manner in which the 
blastopore is closed, they consider the most probable account 
of the matter to be that the blastopore is directed dorsally, 
and is closed by the advance over it of its anterior lip. Xo 
further paper on the development of Amphioxus appeared 
till 1906, when Cerfoutaiue published an exhaustive paper on 
the subject, which, largely because it was illustrated by many 
beautiful figures, has been accepted as decisive by many 
zoologists (9). Cerfontaine is bitterly hostile to the con- 
clusions reached in my work, and it is principally in order to 
see how far his criticisms are justified that I have undertaken 
