EMBEYO OF NUDIBEANCHIATA. 



551 



as the male testis ; but all attempts to trace the excretory duct of either 

 have as yet proved futile. 



(1474.) When the ova of the Nudibranchiate Mollusca are placed 

 under the microscope soon after the extrusion of the spawn, each is 

 seen to consist of a thin transparent case-membrane*, with a round 

 smooth and opake body in its centre (the ovum proper), which is 

 chiefly composed of minute cells enclosed in a vitelline membrane. 

 These ova vary in size from -^ro^h to -g-g-g-th of an inch in diameter. A 

 few hours after the extrusion of the spawn the yelk divides by progressive 

 segmentation until the end of the fifth day, when the division of the 

 cells appears to have reached its utmost limit and the vitelline mass has 

 changed its shape, having become broader at one end, narrower at the 

 other (fig. 276, 2). At the end of the sixth day no additional change takes 



Fig. 276. 



Development of the embryo of a Nudibranchiate Mollusk. 1. Gelatinous coil, in which the 

 ova are imbedded. 2. A portion of the same, magnified. 4, 5. Embryos in different stages of 

 growth. 6. Mature embryo when newly hatched, enclosed in a minute nautiloid shell. 



place in the external form of the ovum, but the cells into which it has 

 divided continue to coalesce, and minute cilia become apparent on the 

 upper surface of the broad extremity. On the eighth day it assumes 

 the form represented at fig. 276, 3, its circumference becomes more or 

 less translucent, and the external layer of cells seems to separate from 

 the rest to form the commencement of the shell (fig. 276, 4), the cilia on 

 the broad extremity become larger and more active in their movements, 

 and traces are observed of the division of this end into ciliated disks : it 

 is now entitled to the name of embryo. 



* This case-membrane is not the homologue of the ordinary egg-shell, seeing that 

 it sometimes encloses two, three, four, or even five ova. 



