487 



ternal tubular membrane, and consisted of a parent cell, containing 

 young ones within it. 



The author here directed attention particularly to the mode of 

 growth in this and the simple Acephalocyst, as compared with that 

 of the next known animal in the class, viz., the Csenurus. He stated, 

 that, in the first (Hydatid), one cell was thrown off from the internal 

 surface of the vesicle, which increased in size by simple dilatation, 

 without any cellular development whatever. The growth of the new 

 form of Acephalocyst was exactly similar, — with this difference, 

 that originally there was a parent cell which formed a number of 

 young cells within it, each of which afterwards became the separate 

 individual, by simple dilatation also. 



The author, after describing the adult Cajnurus, then proceeded 

 to state the observations he had made on its development from the 

 ovule upwards. He stated, that the ovule, when observed within 

 the body of the parent animal, was composed, first, of a germinal 

 spot within a germinal vesicle, and which was enclosed in a yolk of 

 very considerable size. The yolk, again, was surrounded by a very 

 thin layer of albumen, defended by the shell or external covering.* 

 After the ovum escaped from the parent, the germinal spot increased 

 very much in size, and a small clear spot appeared in its centre. 



During the third stage, the germinal spot had increased consider- 

 ably in size, and had become nodulated ; the central spot had also 

 increased in size. 



During the fourth stage, the nodules had become separate cells, 

 surrounding a central cell, which had also a germinal spot within it. 

 This again underwent similar chanjjes, and was succeeded in turn 

 by other crops of cells in like succession. This formation of cells, 

 from a succession of centres, only extended the growth in a lateral 

 direction, the author therefore named this the discoidal period of 

 growth, which again he subdivided into minor stages. 



After describing all the stages of discoidal development, he pro- 

 ceeded to point out a change that took place in the mode of develop- 

 ment of the cells, and which, according to the direction of growth, he 

 named the vertical period of growth. 



During the discoidal pei-iod of development, all the cells were pro- 

 duced from one series or succession of centres ; but during the ver- 

 tical period, instead of one, there were several, each of which form 



* The author lo6ks upon these partu of tin.' ovule as merely analogous 

 to those of the liigher animals. 



