ON THE OUTLOOK OF VERTEBRATE MORPHOLOGY. 81 



The second part of the treatise is devoted more especially 

 to embryology, and opens with an elaborate table of varia- 

 tion in external form during development. The succeeding 

 chapter is a most important one, and deals, among other 

 things, with the first indications of the vestigial hind limb. 

 The lingual musculature, the study of lip-formation in 

 relation to lactation under water, the development of the 

 external ear, and the morphology of the mammary organ, 

 are in order considered ; and the volume closes with a 

 chapter on the teeth, in which, controverting the earlier 

 conclusions of Eschricht, Julin, and Max Weber, the author 

 elaborates his now famous argument for the origin of the 

 Cetacea from heterodont diphyodont ancestors, with accom- 

 panying numerical increase of their teeth by subdivision- 

 on the whole the most revolutionary outcome of his labours. 

 He deals with the variations upon this theme, and shows 

 that while, during the fcetal life of the whalebone whales, 

 the teeth of the milk set alone reach the calcified condition, 

 among the Odontoccetes, calcified milk and successional 

 teeth may, by fusion, give rise to a single complex derivative. 



While there can be no two opinions as to the value of 



Professor Kiikenthal's observations, there are one or two 



side issues which, without going into critical detail, call for 



special comment. His conclusion that the cerebral furrows 



are nutritive depressions which mark the disposition of the 



leading circulatory channels, is of especial interest at the 



present time, when it is doubtful whether the leading fissures 



of the mammalian brain really mark out the boundaries of 



localisable areas of response to artificial stimulation. They 



are the more suggestive in consideration of Cunningham's 



surmise 53 that the transitory fissures, which appear in the 



human foetus, only to vanish as the vault of the cranium 



increases, are of mere passing mechanical origin, and 



of the most careful investigations of Forsyth Major, 64 



which have quite recently shown us that the Lemurine 



brain undergoes, during the growth period, a diminution in 



bulk, if not a structural simplification also, proportionate to 



the rest of the. body, and that have led him to the startling 



conclusion that those Lemuroids and Insectivora, possessed 



6 



