ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 259 



beginning with the embryonic series, five distinct sets of teeth are re- 

 presented in the preniaxillae up to the time when the body-length 

 is 21*2 cm. Each adult "incisor" tooth has two components. The 

 vomerine teeth are in course of suppression, and in most cases are com- 

 pletely absent. A remarkable fact is, that after the completion of the 

 alternating series of teeth, there is a more or less cessation of tooth de- 

 velopment in the main dentigerous regions. Subsequently a renewed 

 formation of teeth takes place from behind forwards, and these are 

 uniform in size. These facts, together with some others observed by 

 Dendy, the author regards as a proof that the long period of incubation 

 is a recent acquisition. Its result has been to render the early dentition 

 functionless, to crowd the second and third dentitions together both in 

 time and space, and to produce fusion of teeth both in the premaxilla 

 and the mandible. The uniform teeth appear to be a recent acquisition. 

 Except the first dentition, all the teeth have a coating of enamel. 



Intermaxillary Tooth in Vipera aspis.* — Dr. H. Martin finds this 

 tooth for some time after birth. It corresponds to that which Sluiter 

 has described in oviparous forms as of service in perforating the egg- 

 shell. But in the viviparous viper it seems to be of no use ; it is not 

 fixed to the bone ; and the mucous layer which covers it is intact after 

 birth. It seems necessary to fall back on the supposition that the tooth 

 in question had a function in ancestral forms. While Rose described 

 two tooth-germs in the same position in Vipera berus, and noted that the 

 left one underwent rapid retrogression, Martin finds three germs, of 

 which the median one persists to form the tooth referred to. 



Development of Pancreas in Mammals.| — Dr. K. Helly finds that 

 the dorsal part of the rudiment of the pancreas originates as an unpaired 

 outgrowth along the suspensory ligament of the gut, and is differentiated 

 from the gut in the caudo-cranial direction. It frequently becomes 

 paired in the later stages of development. The ventral rudiments arise 

 not from the gut, but only from the lateral walls of the primary hepatic 

 duct before this opens into the gut. They are paired in origin, J but the 

 left rudiment degenerates. The right usually becomes further dif- 

 ferentiated, but may also at times become degenerate. The fusion of 

 ventral and dorsal rudiments is accompanied by clear indications of 

 activity in the gland-cells. 



Development of Excretory System in Dipnoi. $ — Dr. E. Semon finds 

 that in his stage 29 the rudiment of the pronephros first appears in 

 Ceratodus, as a solid swelling of the parietal mesoblast near myotomes 5 

 and 6. Later this swelling acquires a lumen, and appears to consist of 

 two segmentally-arranged parts, each opening by a funnel into the un- 

 segmented body-cavity. At the same time a small ventro-lateral portion 

 of the pronephric rudiment is differentiated, and constitutes the " ventral 

 portion of the pronephros " of Furbringer. This becomes continuous 

 with the pronephric duct, which, according to the author, has no 

 ectodermic element. In stage 39 the glomerulus is developed. Its 

 longitudinal extent is determined by the position of the two pronephric 



* Journ. Anat. Physiol., xxxvii. (1901) pp. 80-9 (9 figs). 



t Arch. Mikr. Anat., lvii. (1901) pp. 271-335 (3 pis. and 20 figs.). 



t Zool. Anzeig., xxiv. (1901) pp. 131-5. 



