No. 2.] DEVELOPMENT OF PETROMYZOAT. 275 



further; the broadening and flattening of the cord, the reduc- 

 tion of the canal, the increase of the fibrous bands, and the 

 lateral extension of the cell mass are all more clearly marked. 

 The third stage, of 30 mm. (Fig. 46), shows the changes in the 

 same direction more pronouncedly, especially the increase of 

 the white matter, the broadening and depression of the mass of 

 cells, and the diminution of the canal. In large Ammocoetes 

 (Fig. 47), the cord has assumed the characters found in the 

 adult. Here the cells form a very small proportion of the whole, 

 and the canal has become circular in shape and minute in size. 

 Several intermediate stages between Figs. 46 and 47 might 

 be shown, but those given are sufficient to make the process 

 intelligible. The point to be emphasized is that the features 

 peculiar to the Cyclostomata are of comparatively late appear- 

 ance, while the early stages are in entire accordance with those 

 of the higher vertebrates. 



3. The Cerebrospinal Mentbranes. — Langerhans (22), 

 Ahlborn, and others have shown that cerebral membranes, in 

 the proper sense of the word, are not present in Petromyson. 

 The brain is encased in a soft cellular tissue with firmer internal 

 and external layers, which the older writers named dura and 

 pia mater. On passing from the brain to the spinal cord, 

 certain histological differences in this tissue are noticeable, 

 which need not be dwelt upon here. In advanced embryos and 

 young larvae, indifferent mesoblastic cells gradually enclose the 

 nervous axis, part of which give rise to a dense fibrous envelope 

 (Figs. 34, PI. X; 44, PI. XI), separated from the nervous axis 

 by numerous small rounded cells. In larvae of 20 mm. and 

 upwards in length, branched pigment cells become conspicuous 

 in this tissue, which at the time of metamorphosis becomes 

 greatly increased in bulk. In young larvae (Figs, 45-46), a 

 distinct layer of small spindle-shaped cells may be observed 

 surrounding the spinal cord ; but I have not been able to detect 

 this in older larvae, nor at any stage in connection with the 

 brain. 



II. The Peripheral Nerves. 



The development of the peripheral nerves is exceedingly 

 difficult to follow accurately and continuously, both on account 



