214 EMBRYOLOGY 



fusoriform embrjos, the rhombogen individuals have entered 

 upon the second phase of their development (secondary Nema- 

 togens according to Whitman). 



The formation of the infusoriform embrjos from the germ 

 cell also begins with a process of cleavage, the result of 

 which is an epibolic gastrula (E. van Beneden). However, 

 in this case several cells make their appearance in the centre, 

 at first four large ones (Fig. 100 A, u). Two of these be- 

 come the shell and two the lid of the urn ; whereas four 

 smaller cells, which arise later, supply the four granular 

 cells contained in the urn (Fig. 100 B and C, gr). In the 

 meantime the two highly refractive bodies have made their 

 appearance in the outer layer of the embryo (Fig. 100 A, 1), 

 ;•), and its posterior portion has become covered with cilia. 

 Whereas at first the embryonal cells which become the urn lie 

 side by side, they subsequently alter their position so that 

 the granular cells become enclosed above and below by the 

 lid and shell of the urn. 



Nothing definite is yet known about the significance of 

 the infusorifoi-m embryos. From the fact that they can be 

 kept alive in sea- water for days (E. van Beneden), it was 

 thought that these forms were probably for the purpose of 

 transferring the species from one cephalopod individual to 

 another, where they would develop into a form which, like 

 the vermiform embryos, produces new germs. Besides this 

 vievv, there is a second one, which compares the infusoriform 

 embryos to the male of the Orthonectidae. Van Beneden is 

 inclined to see in the granular and vibratory contents of the 

 urn the homologue of the testis of the Orthonectidae. 

 Whitman several times observed the penetration of infusori- 

 form embryos into nematogen individuals, which is perhaps 

 to be compared to a process of fertilization. 



Related to the Dicyemidse are the Heterocyemidce {Conocyema and 

 Microcycma), observed by van Beneden (No. 2), which also inhabit the 

 appendages of the veins of Octopus and Sepia. Their shape differs from 

 that of the DicyemidiB inasmuch as they do not nearly attain the length 

 that these do, and wart-like structures are present at the anterior end, 

 which can be extended and withdrawn. Nematogen and rhombogen 

 individuals are also distinguished here. Although the vermiform embryos 



