PL A T YEL MINTHES. 1 6 1 



If Agassiz' interesting observations can be trusted we have in this 

 larva indications of a distinct segmentation, which are of some morpho- 

 logical importance, especially when taken in connection with the traces of 

 segmentation found amongst the Nemertiues. 



A further type, with an incomplete metamorphosis, has been observed 

 by Girard (183). It is remarkable for having an uniform segmentation, 

 and for presenting a quiescent stage after passing through a free larval 

 condition with a large upper lip. 



Fresh-water Dendroccela. The development of the fresh-water 

 Dendroccela has been especially investigated by Knappert (No. 186) 

 and Metschnikoff (No. 188). 



The ova are very delicate minute naked cells, which to the 

 number of 4 6 or more become enveloped in a capsule or cocoon 

 together with a large mass of yolk cells derived from the vitel- 

 larium. The yolk cells exhibit peristaltic movements and send out 

 amoeboid processes. Each ovum when laid becomes surrounded by 

 an extremely delicate membrane, which disappears during the course 

 of development. The capsules consist of a spherical case and a 

 stalk. The latter is first emitted from the female opening as a thread- 

 like body. Its free end becomes attached, and then the remainder of 

 the capsule is ejected. 



Impregnation takes place before the formation of the capsule. The 

 segmentation is complete. The ovum first divides into two segments. 

 One of these next divides, forming three segments. There are sub- 

 sequently stages with four, eight, sixteen, and thirty-two segments. 



Metschnikoff 's results on the stages subsequent to the segmentation 

 are not in complete harmony with those of Knappert; but no doubt 

 represent an advance in our knowledge, and I shall follow them here. His 

 observations were made on Planaria polychroa. 



In the earliest stage observed by him the segmentation was already far 

 advanced, but no membrane was present round the ovum. At a later stage 

 the ovum becomes more or less bell shaped or hemispherical, and encloses 

 within its concavity a mass of yolk elements. It is now formed of three 

 concentric layers. An outer layer of flattened cells the epiblast, a 

 middle layer of fused cells the mesoblast, and an inner solid mass of yolk 

 cells the hypoblast. 



At the upper pole is formed the protrusible pharynx (cf. Knappert), 

 which is provided with a provisional musculature and a lumen. By its 

 contractions it takes up the yolk elements which surround the embryo, and 

 the rapid growth of the embryo no doubt takes place at their expense. The 

 embryo gradually loses its hemispherical form, and assumes an elongated and 

 flattened shape. It acquires a coating of cilia by means of which it rotates. 

 On the fifth day it is hatched. 



The alimentary tract long remains solid, even after it has acquired 

 its branched form. The phaiynx becomes withdrawn as soon as the larva 

 is hatched. It loses its provisional muscles, and subsequently acquires 

 a permanent musculature. The young after hatching attach themselves to 

 the body of their parent, on which they feed (?). 



B. E. 11 



