No. 2.] DEVELOPMENT OF PETROMYZON". 285 



retardation, either in the time of their formation or in their 

 subsequent differentiation. This retardation is, doubtless, to be 

 explained by the habits of the larva, which lives buried in sand 

 or mud, and to which the organs of higher sense, more particu- 

 larly that of vision, could be of little or no service. A great 

 increase in the size, complexity, and perfection of the sense- 

 organs takes place at the time of metamorphosis, when the 

 sexually complete animal seeks the clear water and abandons 

 its subterranean habits. 



I. The Sense-Organs of the Skin. — Beard (6), in the beauti- 

 ful observations already quoted, has followed the development 

 of the sense-organs of the skin in the Selachians, and has given 

 a very complete account of their formation in that group. He 

 finds that these organs arise from the epiblastic thickenings 

 which fuse with the roots of the cranial nerves, and, partially at 

 least, give rise to their ganglia. In the case of the lateral line, 

 the epiblastic proliferation, which forms the lateral branch of 

 the vagus, also develops the sensory portion of the skin. In 

 Petroniyzon the history of these organs is apparently quite 

 different. Shipley was unable to observe any rudiment of these 

 organs in connection with the thickenings of the epiblast, which 

 assist in the formation of the ganglia of the central nerves; 

 and, in spite of the most careful and persistent search through 

 large series of sections taken in all three planes, I have had no 

 better success. So far as I can make out, the serial sense- 

 organs in the head, and those of the lateral line, arise quite 

 independently of the cerebral nerves which supply them. 

 Shipley has, however, placed the period of their formation 

 much too late, saying that they had not appeared in his oldest 

 larvae (52 days after impregnation). I have first detected them 

 in larvae of about 7 mm. in length, and here only in the upper 

 lip. Their mode of formation is very simple : a small group of 

 epiblastic cells begins to increase in vertical height, and by 

 pushing aside the surrounding cells, reaches the surface. The 

 increase in height is accompanied by a decrease in breadth, so 

 that the cells have the appearance of protoplasmic rods, which 

 pass directly from the underlying mesoblastic dermis to the 

 surface of the head, while the surrounding indifferent epiblast 

 is made up of several layers of more or less flattened cells. A 

 pit is formed, by a slight separation of the indifferent cells from 



