ZOOtOGY AND BOTANY, MICROSCOPY, ETC. 169 



pigment-bearers into pigment is apparently brought about by specific 

 cell-ferments, which act on the chromatin (the parent- substance of the 

 pigment) when the nuclear membrane disappears temporarily in mitosis, 

 or when chromatin particles are extruded from the nucleus. 



Development of Teeth in Wallaby.* — A. Hopewell-Smith and 

 H. W. Marett Tims have studied the tooth-germs of Macropus billardieri, 

 and give an account of their history. There are some histological 

 peculiarities. 1. The oral epithelium along the alveolar margins is very 

 thick, and heaped up to an unusual extent (like the " Zahnwall " of 

 Ungulates). 2. There is unusual compactness in the stellate reticu- 

 lum, and definiteness in the stratum intermedium of the enamel-organ. 

 3. The enamel shows a precocious development, calcifying as soon as 

 the dentine, or even preceding it. 4. Tliere is abundant evidence of 

 blood-vessels within the enamel organ. 5. There is some, though in- 

 decisive, evidence of the fusion of enamel-organs. 



As to the homologies of the teeth, there seem to be six incisors in the 

 upper jaw, of which the fifth is unrepresented in the earlier stages and 

 has attained but a slight degree of development in the second stage. In 

 the lower jaw there are evidences of five ante-molar teeth. The fourth 

 true maxillary tooth of the upper jaw has a well-marked " concentric 

 epithelial body " lying superficially to it — the last trace of a tooth vestige. 

 It seems to be tolerably certain that the second maxillary tooth will not 

 be able to develop further, thus confirming the opinion of Oldfield 

 Thomas that |jm^ is the missing premolar. The authors incline to the 

 view that the permanent dentition of the marsupials is the permanent 

 dentition of the Eutheria, and that both the deciduous and successional 

 pm.'^ belong to one and the same dentition. 



Alleged Skeletogenous Character of Ectoderm in Vertebrates.! 



Nils Eosen discusses the various attempts that have been made to show 

 that the ectoderm shares in skeleton-making in Vertebrates. There are 

 three questions : — 



1. Are skeletal elements formed in the ectoderm, as stated for some 

 cases by von Kuplfer and by Klaatsch ? 



2. Do skeletal elements sometimes arise by a direct immigration of 

 ectoderm cells, as Klaatsch, Piatt, and others have described ? 



3. Do skeletal elements arise, not by direct immigration, but from 

 ectoderm or endoderm cells which have migrated into the mesoderm at 

 an early stage (Piatt's " mesectoderm " and " mesendoderm") ? 



To answer these questions, Posen has studied the development of 

 cartilage and bone in the trout's skull, in other Teleosts, and in the toad. 

 He has not been able to find any hint of evidence for an affirmative 

 answer. There is no trace of immigration inwards from the ectoderm to 

 form skeleton. Nor has he found any skeletal elements in the ectodermic 

 epithelium itself. Therefore he gives an emphatic negative answer to 

 the three questions stated above, and upholds the old view that the 

 Vertebrate skeleton is mesenchymatous in origin. 



* Proc. Zool. Soc. (1911) pp. 926-42 (1 pi. and 10 figs.). 



t Lund3 Univ. Arsskrift, vi. (1910) No. 7, pp. 1-27 (1 pi. and IG figs.). 



