144 Royal Society : — 



more advanced, this apparent inversion of the cyst has increased, 

 the neck has become longer, and the appearance of disks, hook- 

 lets, and laminated bodies is sufficiently distinct to be perfectly 

 recognizable. The process of development is particularly apparent in 

 the booklets, and perhaps there is no other instance of the growth of 

 an animal tissue which presents such facilities for the examination 

 of the manner in which it is effected. First, because the part of 

 the entozoon on which these organs are formed, is sufficiently trans- 

 parent to admit of examination by the highest magnifying powers 

 without any previous dissection. Secondly, because the material 

 of which they are composed is so characteristic, and so dissimilar 

 to the surrounding parts, that it can be detected in the minutest 

 possible quantities. And, thirdly, as only a few of these booklets 

 are in progress of development at one time, and as these are in all 

 stages of formation, every step in the progress of their growth can 

 be traced from the merest molecule to a perfect booklet. This is 

 important in reference to the general theory of development, as it 

 furnishes an example of the formation of a complete set of organs, 

 on a plan more simple, and at variance with the cell-theory of 

 Schwann and others. Before one of these booklets takes on a 

 recognizable form, it exists as a group of exceedingly refractive 

 particles, all apparently of the same composition, and of a more or 

 less globular form, but of veiy different sizes, some being so minute 

 as scarcely to be visible by one- eighth of an inch lens, others 

 being almost as large as the handle of a perfect booklet, while 

 the rest are of all dimensions between these extremes. The next 

 condition of a booklet is the apparent fusion or coalescence of 

 some of these particles into the hocked part of the organ. Then 

 the handle and tubercles are added, these having been previously 

 formed by the fusion of the smaller particles, and these latter by 

 the coalescence of the minutest and the minuter ones. Before 

 the several parts are perfectly consolidated, their points of junction 

 can be distinguished, and in other groups the fragments corre- 

 sponding to those recently united can be recognized. Directly a 

 booklet is found, it is of its full dimension ; and some of its parts 

 are even larger and more clumsy-looking than in older booklets. 

 The substance of the particles entering into these organs, after 

 they are once formed, undergoes no change in its microscopical 

 characters, but is the same after as before their union. It is im- 

 possible to single out any one particle from the rest, which can be 

 taken for the nucleus of a cell, or for what physiologists would call 

 a nucleated cell; and thus there is nothing which indicates that 

 these organs have been formed by transformation of previously ex- 

 isting cells, but, on the contrary, there is every appearance that 

 their formation is due to the simple coalescence of homogeneous 

 molecules. 



Up to the present point, the facts which I have stated are so 

 obvious, that their accuracy will, I think, not be questioned ; also 

 the interpretation of them is not only that which appears to 

 me the most natural, but is almost self-evident. There remain. 



