PLATYELMINTHES. 195 



(fig. 90) the body was considerably flattened and had approached more to 

 the planarian form. 



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

 indications of a distinct segmentation, which are of some morphological 

 importance, especially when taken in connection with the traces of segmen- 

 tation found amongst the Nemertines. 



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 Dendrocoela. The development of the fresh- 

 water Dendrocoela 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 

 vitellarium. The yolk cells exhibit peristaltic movements and 

 send out amoeboid processes. Each ovum when laid becomes 

 surrounded by an extremely delicate membrane, which dis- 

 appears 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 subsequently 

 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 observa- 

 tions 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 



132 



