LAW OF DEVELOPMENT KNOWN AS VON BAER’S LAW. 37 
but it is maintained that the law is justified by certain remark- 
able features of embryonic similarity which the adults do not 
exhibit, and of which the most important are the presence in 
the chick of pharyngeal clefts, a tubular piscine heart, and a 
similarity in the arrangement of the cardiac arterial system, 
a cartilaginous endo-skeleton, oro-nasal grooves, and a noto- 
chord. NowlI freely admit that these are striking similarities, 
but I question whether they are sufficient to justify the law of 
v. Baer. By themselves, no doubt, they would be sufficient to 
justify that law ; but are there no differences to set off against 
them ? Are there no differences of a morphological value, as far- 
reaching and as striking as these similarities? Let us clearly 
understand the question at issue. V. Baer’s law, as applied to 
the present case, may fairly be held to mean, if it has any 
meaning at all, that whereas the differences between the adults 
are large and important differences of class value, the differ- 
ences between the embryos are slighter and unimportant, and of 
less than class value. Now in no single member of the group 
Craniata is the mesoderm of the head segmented. According 
to our present morphological knowledge, the discovery of an 
animal with cranial segments would be a very remarkable one, 
and would, we might confidently predict, require the establish- 
ment of a class at least separate from all other Craniate 
classes—such is our estimation of the importance of this 
feature. And if to this character was also added the presence 
of a coelomic sac close to the eye, of another in the jaw, and of 
a third near the ear; of an aperture of communication between 
the neural canal and rectum, of kidney tubules opening into 
the muscle-plate coelom as well as into the perivisceral ccelom, 
of a Millerian duct opening into the front end of the Wolffian, 
I do not think that any anatomist would have any doubt about 
the matter. Now it is precisely in these points, amongst 
embryos of different classes of the Vertebrata given by Haeckel in his popular 
works, and reproduced by Romanes and, for all that I know, other popular 
exponents of the evolution theory. As a sample of their accuracy, I may 
refer the reader to the varied position of the auditory sac in the drawings 
of the younger embryos, 
