ZOOLOGY AND BOTANY, MICBOSCOPY, ETC. 389 



is discussed at some length, and it is believed to be the rudiment of a 

 canal which once opened on the surface of the head. In correlation with 

 the retarded development of the eyes the optic lobes do not appear till 

 late in larval life. The cerebellum is formed from the posterior wall of 

 the dorsal fold between the mid- and hind-brains, and long remains very 

 minute. The spinal cord in the later embryonic and early larval stages 

 is like that of the higher vertebrates ; the characteristic flattening is 

 effected during larval life. 



The peripheral nerves are developed much as in Selachians; the 

 olfactory nerves are originally paired, and the optic are at first remark- 

 able for their great length ; the trigeminal has two, and the facial has 

 one ganglion formed from the skin. 



The epidermal sense-organs of the head and lateral line are not 

 developed in connection with the ganglia of the cerebral nerves, or with 

 the lateral nerve, but at a later stage ; this, however, is looked upon as 

 a secondary process. The olfactory organ is at first ventral in position, 

 and is always single and median ; the rotation of the upper lip brings 

 the opening to the dorsal side of the head, and it was probably this 

 condition which produced the coalescence of the primitively paired nasal 

 pits. A glandular organ, resembling that of Jacobson, but having no 

 communication with the mouth, is formed from the postero-inferior 

 portion of the nasal involution. The eye is formed as in other verte- 

 brates, but is remarkable for the very small part of the primary optic 

 vesicle which gives rise to the retina ; the retinal elements appear only 

 just before metamorphosis, and no cornea is present in the larva ; the 

 lens is probably of mesoblastic origin. The ear, in its early stages, is 

 like that of other vertebrates, and the first divergencies appear in the 

 young larva. There is no trace of the horizontal semicircular canal ; the 

 vestibule is divided imperfectly into chambers, and a median appendix 

 is formed. This organ is relatively better developed in the larva than 

 either nose or eyes, and does not undergo such marked changes at 

 metamorphosis. Dr. Scott considers that the sense-organs of Petromyzon 

 do not show degeneration, but rather retardation of development. There 

 are certain minor peculiarities which appear to have been acquired 

 within the Cyclostomatous phylum, but they cannot be regarded as 

 degenerate ; such are the union of the nasal pits, and the development 

 of the naso-palatal canal ; the peculiar structure of the retina ; the 

 absence of the horizontal semicircular canal, the division of the vestibule 

 into chambers, and the presence of the auditory appendix. 



Development of Torpedo ocellata.*— Prof. A. Swaen, in the first 

 part of his memoir, deals with the formation of the gastrula, of the meso- 

 blast, and of the notochord in Torpedo ocellata. He finds that the meso- 

 blast arises throughout the blastoderm from a mixture of epi- and 

 hypoblastic cells. In the anterior part it arises from the special cellular 

 zone which connects the epiblast and hypoblast ; in the posterior part it 

 arises indirectly, in the sense that the zone of cells commences by forming 

 a special layer (the secondary hypoblast), from which alone the mesoblast 

 takes its origin. The notochord is found to be developed from the endo- 

 blast of the digestive tube, on the roof of which a notochordal groove is 

 first formed. The epithelial cells of tho groove are, in consequence of 

 their orientation around the notochordal axis, partly isolated from the 



» Arob. de Biol., vii. (1887) pp. 537-85 (3 pis.). 



