366 LECTURE XXIV. 



raent of the Mammiferous ovum most closely resemble these in 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 pi'ocreation 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 conspicuous changes in the yolk, has been 

 mainl}^ established by collecting the observations that have been made 

 upon the development of the embryo in the diff"erent classes of Inverte- 

 brata, and by the comparison of these with the analogous phenomena 

 observed in the development of the Vertebrate Animal, and which 

 Ave may conclude to characterise the first steps in the formation of 

 the human embryo. 



And since later observations have established what I first observed, 

 and suggested to be a general property of the blood-corpuscle, — viz. 

 its power of spontaneous subdivision into smaller vesicles *, it is 

 highly probable that the preliminary steps to its conversion into a 

 tissue 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 off'er the most conspicuous 

 examples, are the most universal and important in the operations of 

 the living animal, ceases to wear the aspect of exaggeration. 



It is however certain that the most extraordinary consequences of 

 the fissiparous property of the nucleated cell are manifested in the 

 Invertebrate series of animals, of which generation without fecunda- 

 tion of the procreating individual is an example. This phenomenon, 

 which has so much astonished and perplexed physiologists, as mani- 

 fested in the Aphides, becomes perfectly intelligible and reducible to 

 the general law. 



The Monad divides itself before our eyes, constituting two, then 

 four, next eight individuals, and so on. The impregnated germinal 

 vesicle, which the Monad permanently represents in nature, propa- 

 gates, in like manner, by spontaneous fission and assimilation, a number 

 of impregnated cells like itself. Most of these cells are metamor- 

 phosed into the tissues of the growing embryo, but not necessarily all. 

 Certain nucleated cells, the progeny of the primordial one, and in- 

 heriting its powers, may become, without further stimulus, the centres 

 of development of processes like those which have built up the body 

 that contains them : they may bud forth from the stem of the Hydra, 

 and form new individuals by the process of gemmation ; they may 

 bud forth, in like manner, from the larval polype of the Medusa, which 



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

 Transactions, 1840. 



