MUSEUM OF COMPARATIVE ZOOLOGY. 93 



The development of these structures has been studied by Beddard 

 ('8$, p. 450). In the youngest embryos which he examined, the axial 

 portion was already formed, and at that stage it was closely invested by 

 the four retinular cells and two other cells, the hyaline cells. Judging 

 from their positions, Beddard believes that both kinds of cells may con- 

 tribute to the formation of the axial structure, although the fact that 

 this body is squarish in transverse section leads him to conclude that the 

 four retinular cells play the more important part in its formation. Bed- 

 dard regards the axial body as the rhabdome of the immature eye. In his 

 opinion, the rhabdome in the adult is produced by subsequent secretions 

 from the retinular cells, and presents the form of the four rhabdomeres 

 already described. Although these rhabdomeres form the principal part 

 of the rhabdome in the adult eye, he believes that the rhabdome of the 

 earlier stages persists as the axial fibrous structure in the later stages, 

 and constitutes perhaps the greater part of its distal continuation 

 between the rhabdomeres. 



Unless some such explanation of the origin of the axial part of the 

 rhabdome as that proposed by Beddard be accepted, it is difficult to 

 understand how the fibrous portion could arise as a secretion ; for in the 

 adult the proximal portion of it is touched by neither retinular nor 

 hyaline cells. 



Granting for the moment the adequacy of Beddard's explanation of 

 the origin of the axial part, we are still confronted by what appears to 

 me to be unparalleled in the structure of the eyes in Arthropods, namely, 

 an ommatidium which produces two distinct rhabdomes. This may not 

 be an impossibility, but if it occurs at all, it is certainly exceptional. 



I believe, however, that the so called axial part of the rhabdome in 

 Serolis is capable of another interpretation, against which the objections 

 already suggested cannot be urged. That the axial portion terminates 

 proximally on the basement membrane has been fairly well established 

 by Beddard. The distal termination of it, however, has not been so 

 clearly made out. It is my belief that the axial structure is directly 

 continuous distally with the cone cells; in other words, that this struc- 

 ture is to be regarded as a proximal extension of the cone cells, not as 

 a part of the rhabdome. The termination at the basement mem- 

 brane of this prolongation of the cone cells, as observed by Beddard, 

 is perfectly consistent with the interpretation which I have suggested, 

 and makes the condition in Serolis similar to that in Homarus, where 

 the fibrous ends of the cone cells also terminate on the basement mem- 

 brane. That the fibrous structure should be present in the embryo of 



