DEVELOPMENT OF EMBKYO. 67 



Cysticercus pisiformis, but which may serve as a general scheme, 

 there are certain variations according to the species. In 

 Cysticercus celluloses the processes are exactly the same as those 

 just described, but the cephalic process does not hang down 

 perpendicularly, but obliquely, into the caudal vesicle ; and in 

 this the formation of the vessels and caudal vesicles are recog- 

 nised from the first production of the foundation of the head. 

 The latter also applies to Cmnurus, in which each individual head 

 is formed in a separate inverted portion of the general embryonal 

 vesicle, to which a sort of perforation leads from the outside. 

 I have sometimes thought that I could perceive, in this case, the 

 same mode of formation of the head as is above indicated occurring 

 here in the anterior thickened end of the little body enclosed in 

 the inverted portion of the vesicle, — consequently, an inversion 

 of the cephalic process towards the interior of the structure 

 contained in the inversion. Frequently, I might say in most 

 cases, I could not detect any such inversion of the cephalic 

 structures in the above-mentioned internal body; but I observed, 

 at the anterior end of the internal body, only a retraction of 

 the rostellum with its hooks. The position of the hooks was 

 as in the other vesicular worms, that is to say, the extremities of 

 the shafts turned forwards and outwards, and the apices of the 

 hooks inwards and backwards; and for the same reason the 

 suckers were here also on a level with the circlet of cilia, and 

 placed laterally from them, whilst in the other vesicular worms 

 the suckers do not stand on one side of, but behind the rostellum, 

 as long as the head is still inverted (PI. I, fig. 9). 



In Cysticercus fasciolaris the same formation occurs, at first, as 

 in Cysticercus pisiformis, but subsequently the head and body 

 are protruded from the caudal vesicle. 



In all other species, even when there is no formation of a 

 caudal vesicle, as in the platycercal and acercal forms, especially 

 of cold-blooded animals, the formative process is fundamentally 

 the same. For even in these the true tape-worm head is developed 

 in the interior of the embryonal vesicle, at all events originally, 

 in the form of a process, which is produced from the walls of the 

 top cavity of the embryo, or from its hinder part. The form 

 under which most of these formations have been hitherto observed, 

 is not so much the primary as a secondary form, analogous to that 

 which we have seen in Cysticercus fasciolaris. At the same time 

 the head has already more and more attained the aspect of the 



