1887.] 



ON CYCLESTHERIA HISLOPI. 



45 



IV. 



Ontogeny. 



a. Development of the Summer-eggs. 



The ova just received within the incubatory cavity of the 

 parent (see Pl. III, fig. 1) are spherical bodies, measuring about 

 0.35 mm. in diameter, and exhibit all the characters of true sum- 

 mer-eggs, the dark-green yolk being finely granular and sur- 

 rounded by a very thin and pellucid egg-membrane. The number 

 of ova at once deposited varies considerably and, as in the Clad- 

 ocera, is generally far greater with individuals of the first gene- 

 ration than with those of the succeeding generations, amounting 

 in the former not rare ly to 30 — 35 in all. They are, as 

 above stated, kept in place by the aid of the cirrus-like append- 

 ages proceeding from the upper prolongation of the exopodites of 

 the middle pairs of legs, and are often found firmly adhering to 

 them isee Pl. VI, figs. 1, 2). Moreover, the young contained 

 within the incubatory cavity, even when comparatively developed, 

 exhibit a most regular arrangement, the heads invariably turning 

 downward, the body extending upward and as a rule with the 

 ventral side directed anteriorly (see Pl. I, fig. 3, Pl. II, fig. 1). 

 On closer examination it will appear that this peculiar dis- 

 position is due to the cirri, which still adhere to the embryos 

 by the aid of a thread-like ligament, viz., a remnant of the egg 

 and the embryonic membraues, still connected with the forepart 

 of the embryos. 



