64 EMBRYOLOGY 



to the ventral side by a dorsal accumulation of nutritive entodermal 

 cells. This condition affords a transition to the larvas of Agalma, Crys- 

 tallodes, and Atorybia, in which, by an accumulation of still greater 

 masses of large nutritive cells on the dorsal side of the larval body, a 

 structure almost like a yolk-sac may be developed (Crystallodes). 



The development of Agalma has been described by Metschnikoff 

 (No. 13) and Fewkes (No. 66). The ciliated planula-stage here retains 

 the spherical shape of the egg, but soon exhibits a thickening of the 

 ectoderm at one spot. At this place, which corresponds to the subse- 

 quent ventral side of the embryo, an accumulation of small cells soon 

 takes place, which forms a second cell-layer under the ectoderm (Fig. 26 

 A, en). Both layers separate somewhat from the underlying large 

 nutritive cells, thereby forming the gastral cavity (gi)), while the pro- 

 jection arising in this way is constricted off from the rest of the larval 

 body by a circular furrow, and is to be recognized as the fundament of 

 the first primary hydrophy Ilium (Fig. 26 B, d). It develops further by 

 the secretion of a gelatinous mass (g), lying between the ectoderm and 

 entoderm, which soon increases greatly, so that the entodermic diver- 

 ticulum extending into the hydrophillium becomes a comparatively small 

 plug-shaped organ. A short time after the establishment of the primary 

 cap-shaped hydrophillium, the pneumatophore is formed as an ecto- 

 dermic ingrowth (Fig. 26 B and C, p), which is soon surrounded by an 

 ectodermic duplicature, while the pneumatocyst is developed inside of it. 

 Next there bud forth on the ventral side two new fundameifts of hydro- 

 phillia (Fig. 26 C, d^, d^), which develop into the heteromorphous, 

 leaf-shaped, serrated larval hydrophillia, and meanwhile a new ventral 

 bud is developed into the provisional tentacle (/). By the enlargement 

 of the gastro vascular cavity (gv), the remains of the larval body, which 

 is filled with nutritive cells, are gradually metamorphosed into the poly- 

 pite. The further development is accomplished by the loss of the 

 primary cap-shaped hydrophillium, which is replaced by a circle of 

 foliaceous, likewise provisional, hydrophillia, so that in this way a larval 

 stage is reached which resembles the condition which persists throughout 

 life in Atorybia (Claus). 



The development of Physophora, which in general is like that of 

 Halistemma, is also characterized by the early development of a larval 

 hydrophillium which subsequently disappears, the earliest fundament of 

 which, as it seems, precedes that of the pneumatophore. In the further 

 progress a larva is developed in which the bilateral hydrophillium, which 

 is provided on one side with a fissure, envelops like a spathe the funda- 

 ment of the polypite, the pneumatophore. and the provisional tentacle. 

 The general resemblance of this larva to certain bilaterally symmetrical 

 medusffi (Hybocodon) has already been pointed out by Haeckel (No. 

 68), and later by Balfoub, and has been made use of in support of the 

 medusa theory (see p. 73). 



According to this view, the larva of this stage would represent an in- 

 dividual of a medusoid organization, in which the manubrium of the 

 medusa would be represented by the polypite, and the umbrella of the 



