STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 91 



vessel placed at the insertion of the iris. The membrane 

 itself is composed of a hyaline ground substance with 

 numerous nuclei. 



In the developmental section devoted to the eye the main 

 subject dealt with is the nature of the mesoblastic structures 

 entering the cavity of the optic cup through the choroid 

 slit. It is shown that a large non-vascular mesoblastic 

 process first enters the optic cup, and that together with the 

 folded edge of the choroid slit it forms a rudimentary and 

 provisional processus falciformis. At a later period an 

 artery, bound up in the same sheath as the optic nerve, 

 enters the optic cup, and the vascular membrane found in 

 the adult then becomes developed. 



The Suctorial Disc. — The structure of a peculiar larval 

 suctorial organ, placed at the end of the snout, is described, 

 and the organ is shown to be formed of papillae composed of 

 elongated epidermic cells, which are probably glandular 

 (modified mucous cells), and pour out a viscid secretion. 



Muscular System. — The lateral muscles of Lepidosteus 

 are shown to differ from those of other fishes, except the 

 Cyclostomata, in not being divided into a dorso-lateral and 

 ventro-lateral group, on each side of the body. 



Vertebral Column and Ribs. — This section of the paper 

 commences with a description of the vertebral column and 

 ribs of the adult. In this part special attention is called to 

 a series of cartilaginous elements, placed immediately below 

 the ligamentum longitudinale superius, w hich appear to have 

 escaped the notice of the anatomists who have previously 

 worked at Lepidosteus. These elements are shown to be 

 intervertebrally situated. 



With reference to the ribs the authors point out that for 

 the greater part of their length they course along the bases 

 of the intermuscular septa, immediately external to the 

 peritoneal membrane, but that their free extremities bend 

 outwards and penetrate between the muscles along the 

 intermuscular septa till they nearly reach the skin. 



In the embryological part of this section a detailed account 

 is given of the development of the vertebral column, of which 

 the following is a summary : 



There is early formed round the notochord a meso- 

 blastic investment which is produced into two dorsal and 

 two ventral ridges, the former uniting above the spinal 

 cord. Around the cuticular sheath of the notochord an 

 elastic membrane, the membrana elastica externa, is next 

 developed. The neural ridges become enlarged at each 

 intermuscular septum, and these enlargements soon become 



