ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 5i3 



similarity in the mode of its development and of the optic vesicle, 

 that is, by a hollow outpushing of the central wall ; on the origin and 

 connection of the epiphysis with the optic region of the brain, and 

 especially with the thalamus opticus ; on the morphological resem- 

 blance between the organ and the primitive optic vesicle ; on its 

 almost peripheral position in Petromyzontes, Selacbii, and Ganoids, as 

 well as its completely peripheral position (outside the skull and at 

 the same level with tlie eyes) in the Amphibia ; and, finally, on the 

 primitive connection, detected by Van Wijhe, between the epiphysis 

 and the neural ridge. 



The author, regarding the pineal gland as a frontal eye, thinks it 

 justifiable to compai-e it with the unpaired eye of the Tunicata, and 

 possibly also, of AmpMoxus. 



Segmentation of the Vertebrate Body.* — F. Ahlborn, after a 

 resume of the theory of Gegenbaur as to the composition of the verte- 

 brate skull, points out that an advance was made when later investi- 

 gators discovered evidence in favour of the mesoraeres of the head ; 

 Gotte, who was the first in this line of inquiry, recognized four 

 segments in the cephalic region. The author's own observations on 

 Petromyzan led him to the conclusion that the first two spinal 

 nerves correspond to three mesomeres, and that the first neuromere 

 nearly corresponds to the fourth and fifth mesomeres. From this it 

 seems to follow that the first three myocommata were innervated not 

 by a spinal but by a cerebral nerve ; and that in the hinder part of the 

 head of Petromyzon the parts of three true mesodermal segments are 

 still retained. A further inquiry shows that the three first spinal 

 nerves of Petromyzontes and of anurous Amphibia are completely 

 homologous, and that the first cervical vertebra of the Amphibia 

 corresponds to the fourth myocomma of the lampreys; if this 

 result be correct it follows that the three first myomeres of the 

 lampreys — which we have already recognized as typical cephalic 

 segments — are homologous with the three hinder segments of the skull 

 of the Anura. This remarkable agreement is further supported by 

 the close systematic relation between these two groups which is 

 spoken to by the large number of characteristics that they have in 

 common. 



Gotte was followed by Balfour who attacked the problem by the 

 road of the developmental history of the Elasmobranchii, and domon- 

 Htrated the presence of a somatoplcure and a splanchnopleure in the 

 mesodermal elements of the head, as of the trunk ; this considerable 

 support to the doctrine of the metamerism of the head by the dis- 

 covery of the head-cavities was succeeded by Marshall's investiga- 

 tions, which resulted in showing that metamerism first appeared in 

 the ventral (or branchial) portion only, and by Van Wijhe's work 

 along the lines of the same theories. The studies of the last- 

 mentioned anatomist lead to the conviction that the mesomerism, 

 which is independent of the branchiomerisra, as Marshall proved for 

 the anterior, is true for the whole of the ccidialic tract ; it is a 



* ZcitKchr. 1. WisK. ZooJ., xl. (1««1) i-i). UO'J-yo. 



