No. 3.] STUDIES ON CEPHALOPODS. 253 



Agassiz and Whitman have most fully discussed the impor- 

 tance of this question, while Hallez was led to the formulation 

 of an important law in regard to the orientation of insect ova. 

 Blochmann and Wheeler have described the early exhibition of 

 the bilateral arrangement in insect ova, extending even to the 

 distribution of yolk-granules. 



Several others may be mentioned whose work bears more or 

 less on this important problem. Some of these we will examine 

 more in detail later. It may be pointed out, however, at present 

 that in the study of segmentation of the ovum, a mere quanti- 

 tative enumeration of blastomeres at the successive stages of 

 egg-cleavage alone, has ceased to be a matter of any great im- 

 portance. 



Van Beneden and Julin ^ have demonstrated in the ^^g of 

 Clavellina the existence of a bilateral axis before it begins to 

 segment. The anterior and posterior extremities, the left and 

 right sides, the dorsal and ventral aspects of the larva can be 

 definitely pointed out in the unsegmented ovum. The first 

 furrow of cleavage coincides with the median plane of the 

 bilateral larva, the left half of the larval organization being 

 derived from the first primary blastomere which lies on the left 

 side of the first furrow of cleavage, and the right half of the 

 body, from the right cleavage sphere. Properly speaking, the 

 body of the larval Clavellina consists of two identical halves 

 which meet in the median line. All structures in the body are 

 paired, or have double origin corresponding to the bilateral 

 symmetry of the ovum. The median structures, such as the 

 notochord, digestive tube, are derived from two sources, — the 

 right and left halves of the body, — although later, one side of 

 the body may be invaded by the cells from the other side. 



Chabry,^ who worked on Ascidia aspersa, has confirmed the 

 results of the above-mentioned authors, and has established 

 with certainty the primitively double character of such median 

 structures as the eye, otolith, notochord, the atrium, and the 

 organ of fixation. He thinks that the eye in the adult is de- 

 rived from the cells situated in the anterior end of the right 

 half of the body of the larva. 



1 Ed. van Beneden et C. Julin : La segmentation chez les Ascidiens et ses rapports 

 avec V organisation dela larve. Archives de Biologie, Tome V, 1884. 



2 La segmentation chez les Ascidies simples. Jour. Anat. et Physiol., T. 20, 18S5. 



