LAW OF DEVELOPMENT KNOWN AS VON BAEr's LAW. 77 



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. Now I freely admit that these are striking similarities, 

 but I question whether they are suflficient 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 ofi' against 

 them ? Are there no diflPerences 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 diff'erences 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 coelom, 

 of a Miillerian duct opening into the front end of theWolfiian, 

 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. 



