336 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



of the typically apical portion. In a section exactly in the mid-line of 

 the eye (Fig. 12, Plate LIV) we would see that the apical portion of the 

 eye does not there approach so far toward the base of the eye, and that 

 beneath the rod cells of the basal region are pigment cells like those seen 

 in the section drawn. This eye, then, in spite of its considerable modifi- 

 cation, still conforms to the usual plan ; and its manner of innervation 

 exactly resembling that found in the large eye of the chain Salpa run- 

 cinata-fusiformis, shows that in this species also we have a reversal of 

 position in the developing eye. The rod cells are columnar, with their 

 outer nucleated ends thin-walled and the cell walls of their inner ends 

 unevenly thickened. 



It may seem that possibly the irregular shape of the rod cells seen in 

 the sections of the eyes of the chain individuals of certain species, e. g., 

 Salpa hexagona, Salpa costata-Tillesii, Salpa cordiformis-zonaria, is due 

 to the cells being cut obliquely. I would say that I have cut sections of 

 each eye in three planes and find the same irregularity of shapes in all. 



I realize the incompleteness of the histological observations given in 

 the preceding pages. I cannot, however, hope to make them more com- 

 plete until I am enabled to study fresh material by maceration methods, 

 the use of which is so essential for gaining a right understanding of the 

 shape of the cellular elements, and especially of the manner of inner- 

 vation. I hope at some future date to make a more complete contribu- 

 tion to the knowledge of the histological elements of the nervous system 

 and eye of salpa. 



THEORETICAL. 



We have seen in the foregoing descriptions that the eye of the solitary 

 form has a very uniform structure throughout the different species ; the 

 condition in Salpa cylindrica being probably typical. There are but two 

 noteworthy variations : the first is the imperfect development of the rod 

 cells in Salpa runcinata-fusiformis ; the second is the slight departure 

 from the typical horseshoe-shape which is found in Salpa cordiformis- 

 zonaria. 



We have also seen that the eye of the chain form passes through a 

 stage in its development when it resembles the eye of the solitary form. 

 This is observed even in the group with the most modified eye, the cyclo- 

 salpas. 



In the chain forms of the different species there is a very great 

 degree of variation in the structure of the eyes ; yet it is not difficult to 



