102 



EMBRYOLOGT 



a whole series of individuals remains enclosed by a common theca, 

 whereas the septa are placed pei'pendicular to the direction of the tortuous 

 valleys extending between the theca; (Meandrina). 



In the stone corals also, budding and fission may lead to the foniiation 

 of individuals which separate from the parent and live independently. In 

 Blastotrochus thei-e are lateral buds that separate, whereas in Flabellum 

 a kind of transverse division occurs. The young stages of the Fungidas 

 form small coral stocks from which the solitary forms, which become 

 sexually mature, are abstricted by transverse division. Since one and 

 the same branch may undergo this process of transverse division several 

 times, the resemblance to the strobilization of the Seyphozoa is very 

 striking. Here also there is a true alternation of generations (Semper, 

 No. 93). 



III. SCYPHOMEDUS/E. 



Of the forms belonging here the Liicernarida} and Charyb- 

 deidce are contrasted with the Discophora proper. While the 

 embrjologj of the latter has been repeatedly investigated, 

 we have as yet only a fragmentary knowledge of the two 

 groups first named. 



Lucernaridae. — Fol and Koeotxeff have given accounts of the larvje 

 of the Lucernarians. The development from the egg has been more 

 thoroughly investigated by Kow.\levsky (No. 108), whose results have 

 recently been confii-med by R. S. Bergh (No. 101). After the egg and 

 sperm have been discharged into the water fei-tilization takes place, at 

 the completion of which the egg retracts somewhat from the vitelline 

 membrane. Two polar globules are formed, and then the fu-st cleavage 

 furrow arises. By means of total and equal cleavage a multicellular stage 

 is formed, wliich presents no cleavage cavity. The pointed ends of the i^ris- 

 matic cells meet at the centre. An accumulation of entoderm cells now 

 takes place inside this so-called morula ; this is accomplislied by a contri- 

 bution of elements from a definite region of the egg, so that the production 

 of the entoderm here seems to apijroach the type of polar ingression. 

 KowAXJEvsKY believes that it is chiefly a transverse division of the pris- 

 matic cells in tlris region that leads to the contribution of entodermal 

 elements ; however, simple ingression is not wholly excluded. The 

 bilaminar stage resulting from tliis is at fu-st completely spherical (Fig. 

 49 A), but soon elongates in the direction of the future chief axis (Fig. 

 49 B). The entoderm cells meantime become vacuolated, and arrange 

 themselves more and more in a single row, so that there results from this 

 a rod-like planula, which, like that mentioned for iEginopsis (p. 57), 

 resembles a detached hydroid tentacle (Fig. 49 C). This planula of the 

 Lucernaridae is not ciliated, but creeps slowly about with worm-like 

 movements. The first nettling cells are developed a its iDosterior end. 

 Preparatory to assuming the polypoid form, it eventually attaches itself 



