508 ON TWO REMARKABLE SPOROCYSTS, 



thin layer composed of a small number of flattened cells which 

 become completely united at their edges. This investment 

 persists to a comparatively late stage in the development of the 

 Cercaria, onl}' disappearing as the cuticle becomes formed. 



Schwarze's account (13) of the development of the investing 

 layer in Cercaria armala agrees closely with what I have observed. 

 I cannot, however, follow him in his opinion that the embryonic 

 investment gives rise to the cuticle of the Cercaria. The invest- 

 ment in question is an excessively thin membrane which at no 

 time contains more than about half-a-dozen nuclei. As the 

 embryo grows, this simph'' becomes stretched, and in early tailed 

 stages appears quite loosely connected with the underlying la3^ers. 

 I have very little doubt that it is the laj^er of cells which, at the 

 period when the tail is being formed, becomes arranged in a regular 

 manner beneath the original investing layer, that secretes the 

 cuticle. 



Heckert"^ states that in embryo Cercarise of a diameter of 

 0-075 mm. a second investment similar to the first appears beneath 

 it; and Loos (9) observed a similar phenomenon in the develop- 

 ment of Rediae (letter to Braun quoted in foot-note, 3, p. 818). 

 There is no appearance of this second investing layer in the case 

 of the Cercaria from Mytilus latus. 



Small nuclei with deeplj^-staining, apparently homogeneous,' 

 plasma, similar to those described by Schwarze (10) in Cercaria 

 armata as giving rise to the rudiment of the reproductive system, 

 appear before the embryo leaves the ovary. But these have no 

 definite arrangement, occurring, frequently two together, here 

 and there, both near the surface and more deeply, instead of 

 forming a central mass as stated by Schwarze. They are most 

 probably merely phases in the histor}' of the ordinary meristem 

 nuclei, and have no special histogenetic significance. 



* I only know this paper (' Untersuchungen iiber die Entwickelungs- und 

 Lebensgeschichte des Distomum macrostomum.' ' Bibliotheca Zoologica,' 

 Leuckart u. Chun, Heft 4), as quoted by Braun (3). 



