CONCLUDING SUMMAET. 643 



tition is rightly understood, the multiplication of similar parts for the 

 repetition of the same actions is at once appreciated as essentially 

 the more simple, as well as the inferior, condition, compared with the 

 assemblage of less numerous parts in the same body with different 

 offices, and with prospective arrangements that enable them to 

 combine their different powers for definite ends. 



The lowest Invertebrata resemble locomotive cells : they propagate 

 by spontaneous fission and grow by assimilation ; sometimes they 

 exhibit their geometrically multiplied divisions to a certain extent 

 within a common capsule. The earliest phenomena in the develop- 

 ment of the mammiferous ovum most closely resemble the common 

 mode of multiplication of the Gonium and Volvox. Like phenomena 

 have been observed in the vitelline germ of the frog and the fish. 

 But the universality of the phenomena of spontaneous division, of 

 the fissiparous procreation of nucleated cells, and of their growth by 

 assimilation and coalescence round definite centres, as the properties 

 of the primordial germ in all animals which produce the most conspi- 

 cuous changes in the yolk, has been mainly established by collecting 

 the observations that have been made upon the development of 

 the embryo in the different classes of Invertebrata, and by the 

 comparison of these with the analogous phenomena observed in the 

 development of certain Vertebrata which we may conclude to cha- 

 racterise the first step in the formation of the human embryo. 



And since later observations hare established what I believe 

 myself to have first observed, and suggested to be a general property 

 of the blood-cell, — viz. its power of spontaneous subdivision into 

 smaller centres prior to its solution*, it is highly probable that the 

 preliminary steps to its conversion are the same as those of the 

 nucleated cell which constitutes the germ of the entire animal ; and 

 the proposition that the phenomena of spontaneous fission, of which 

 the Monads offer the most conspicuous examples, are the most 



mate yery much to the nature of the vegetable world. But there are single 

 instances that seem wholly to destroy this gradation. Lyonnet has discovered a 

 far greater variety of parts in the caterpillar of the willow-butterfly than we can 

 observe in many animals of the largest dimensions ; and amongst the microscopic 

 insects in particular, we see a prodigality of machinery, subservient to the various 

 purposes of the contracted life of the little animal, in the structure of which nature 

 appears to be ostentatious of her power of giving perfection to her minutest works." 

 — Youxg's Lectures on Natural Philosophy, 4to., p. 608. My predecessor in the 

 Hunterian chair adopted the same view. " There are 500 muscles attached to this 

 hard ring, whicb passes round the body of the Willow-caterpillar, each muscle 

 having its nerve. Now, if 1 take this opportunity of making a comparison, 

 let me ask whether there be any part of man which presents a complication 

 equal to this? " — Sir Charles Bell, Hunterian Lectures, Lancet, 1833, p. 284. 



* Medical Gazette, November 13. 1839. Dr. Martin Barry's Paper in Philos. 

 Transactions, 1840. 



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