98 ERNEST WARL'EN. 



and gonophore is more granular than the rest of the endoderm. 

 The outer surface of the ectoderm of the embrj'o (fig. 17, ect.^), 

 is not sliai'ply marked off from the placental tissue [pi. t.), 

 and there appears to be the most intimate contact. Both 

 rounded and stellate cells of dense protoplasm are seen in this 

 placental tissue. Between the endoderm {end.i) and the 

 ectoderm {ect.i) of the embryo a faint line of division can be 

 seen, and this is the beginning of the mesogkea. 



Such is the appearance of the embryo in a section cut in 

 the plane of flattening of the gonangium. In a vertical 

 section at right angles to this plane the planula bears a 

 different aspect. The sides of the embryo are squeezed flat 

 against the perisarc of the gonotheca with no gonangial 

 ectoderm between (fig. 18). Laterally the embryo is very 

 thin-walled, and both the ectoderm and endoderm layers are 

 one cell thick, while on the other hand these layers are two 

 or more cells thick at the distal and proximal ends. The 

 blastocccl {Bl.) is roughly rectangular in section. 



Thus the embryo at this stage is of a curious shape. It is 

 thick-walled everywhere (fig. 19), except laterally (fig. 18), 

 over the flattened sides of the gonotheca, where the walls are 

 very thin. 



The material has not permitted an examination of a later 

 stage, but it is probable that the planula in the above con- 

 dition is nearly ready to burst out of the gonotheca, for the 

 gonotheca which Avas immediately below, and therefore 

 slightly older, was empty, and the operculum absent, doubt- 

 less through the bursting out of the planula. 



Summary. — Here we have an interesting case of the 

 nourishing of the developing embryo. In the great majority 

 of Invertebrata the ovum becomes charged with sufficient food 

 or yolk-material to enable developinent to proceed far enough 

 for the young creature to be able to provide itself with food. 

 In the present hydroid the egg remains quite small and is 

 never pi^ovided with a perceptible quantity of yolk, but it 

 segments, and development takes place, in a kind of maternal 

 placental tissue which supplies the embryo with food during 



