102 EMBRYOLOGY 



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

 whereas the septa are placed peipendicular to the direction of the tortuous 

 valleys extending between the theca? (Meandrina). 



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

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

 Blastotrochus there are lateral buds that separate, whereas in Flabellum 

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

 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 Scyphozoa is \evy 

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

 No. 93). 



III. SCYPHOMEDUS/E. 



Of the forms belonging here the Lucernandce and Charyb- 

 deidce are contrasted Avith the Discophora proper. While the 

 embryology of the latter has been repeatedly investigated, 

 we have as yet only a fragmentary knowledge of the two 

 groups first named. 



Lucernaridse. — For. and Kokotneff have given accounts of the larvte 

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

 thoroughly investigated by Kowalevsky (No. 108), whose results have 

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

 sperm have been discharged into the water fertilization takes place, at 

 the completion of which the egg retracts somewhat from the vitelline 

 membrane. Two polar globules are foi-med, and then the first cleavage 

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

 is formed, which presents no cleavage cavity. The pointed ends of the pris- 

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

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

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

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

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

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

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

 bilaminar stage resulting from this is at first completely spherical (Fig. 

 41J A), but soon elongates in the direction of the future chief axis (Fig. 

 4'J Jl). 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 iFiginopsis (p. 57), 

 resembles a detached hydroid tentacle (Fig. 4'J C). This planula of tiie 

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

 movements. The first nettling cells are developed a its i^osterior end. 

 Preparatory to assuming the polypoid form, it eventually attaches itself 



