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TRANSACTIONS OF SECTION D. 839 
vertebrates, that curious tubular communication between the central canal of the 
nervous system and the hinder end of the alimentary canal that is conspicuously 
present in the embryos of lower vertebrates, and retained in a more or less dis- 
guised condition in the higher groups as well. 
The neurenteric canal was discovered by that famous embryologist Kowalevsky 
in Ascidians and in Amphioxus. He drew special attention to the occurrence of a 
stage in both Ascidians and in Amphioxus in which the larva is free swimming 
and in which the sole communication between the alimentary cavity and the exterior 
is through the neuventeric canal and the central canal of the nervous system ; and 
suggested + that animals may have existed or may still exist in which the nerve 
tube fulfilled a non-nervous function, and possibly acted as part of the alimentary 
canal; a suggestion that has recently been revived in a somewhat extravagant 
form. 
A passage of food particles into the alimentary cavity through the neural tube 
has not yet been seen, and probably does not occur, as the larva still possesses 
sufficient food yolk to carry it on in its development. It is therefore permissible 
to hold that the neurenteric canal may be a mere embryological device, and devoid 
of any deep morphological significance. 
The question of variation in development is one of very great importance, and 
has perhaps not yet received the attention it deserves. We are in some danger of 
assuming tacitly that the mode of development of allied animals will necessarily 
agree in all important respects or even in details, and that if the development of 
one member of a group be known, that of the others may be assumed to be similar. 
The more recent progress of embryology is showing us that such inferences are not 
safe, and that in allied genera or species, or even in different individuals of the same 
species, variations of development may occur affecting important organs and at 
almost any stage in their formation. 
Great individual variations in the earliest processes of development, z.e., the 
segmentation of the egg, have been described by different writers. 
In Renilla, Wilson found an extraordinary range of variation in the segmentation 
of eggs from which apparently identical embryos were produced. In some cases 
the ege divided into two in the normal manner; in other cases it divided at once 
into eight, sixteen, or thirty-two segments, which in different specimens were 
approximately equal or markedly unequal in size. Sometimes a preliminary change 
of form occurred without any further result, the egg returning to its spherical 
shape, and pausing for a time before recommencing the attempt to segment. Segmen- 
tation sometimes commenced at one pole, as in telolecithal eggs, with the formation 
of four or five small segments, the rest of the egg breaking up later, either simulta- 
neously or progressively, into segments about equal in size to those first formed : 
while lastly, m some instances segmentation was very irregular, following no 
apparent law. 
It is noteworthy that the variability in the case of Renilla is apparently 
confined to the earliest stages, for whatever the mode of segmentation, the embryos 
in their later stages were indistinguishable from one another. 
Similar moditications in the segmentation of the ege have been described in the 
oyster by LGrooks, in Anodon and other Mollusca, in Hydra, and in Lumbricus, in 
which last Wilson has recently shown that marked ditferences occur in the eggs 
even of the same individual animal. In the different species of Peripatus there 
appear also to be considerable variations in the details of segmentation. 
In the early embryonic stages after the completion of segmentation very consider- 
able variation may occur in allied species or genera. Among Ccelenterates for: 
instance the mode of formation of the hypoblast presents most perplexing modifica- 
tions: it may arise as a true gastrula invagination; as cells budded off from one 
pole of the blastula into its cavity; as cells budded off from various parts of the 
wall of the blastula; by delamination or actual division of each cell of the blastula 
Wall; or it may be present from the start as a solid mass of cells enclosed by the 
Rul i rn 
‘A. Kowalevsky, ‘Weitere Studien iiber die Entwickelungs-Geschichte des. 
Awphioxus lanceolatus; Archiv fiir mikroshopische Anatomie, Ba. xiii. 1877, p. 201.. 
