OVUM. 



37 



gemmation, or simple sprouting of the parent 

 texture. We cannot be too cautious in 

 making wide generalisations in regard to 

 phenomena so various and so imperfectly 

 known as those of alternate or intermediate 

 generation are ; and the only safe course in 

 the progress of such inquiries is to apply 

 terms to the phenomena which are no more 

 than the exact expression of what is well 

 ascertained regarding their nature. Now, who 

 that has observed or studied the history of the 

 two states of the Salpians, and the relation in 

 which they stand to each other, can hesitate 

 to admit that two dissimilar generations alter- 

 nate, and that a different generative process 

 has taken place for the production of each ? 

 or who, knowing the relation subsisting be- 

 tween the fixed marine polype and its free 

 moving medusoid offspring, or that between 

 the larger medusae and their compound polype 

 stocks, would deny to each of these series of 

 beings the attributes of distinct individuals, or 

 regard the productive acts by which they take 

 their origin in any other light than as two dis- 

 similar kinds of generation ? But this seems 

 scarcely more than a question of words. It 

 is important rather to notice that while some 

 of the polypes now alluded to multiply their 

 like (that is polype-forms) by budding, their 

 medusoid progeny also occasionally produce 

 their like (or similar medusoidsj by gemma- 

 tion ; and surely it is expedient to regard as 

 somewhat different that production of distinct 

 medusoid individuals from a polype stock, 

 which is an advance in its stage of being, and 

 which gives rise to an animal different in 

 structure, mode of life, and functions, from its 

 parent, from that kind of production which 

 is no more than the repetition of the parental 

 form or the extension of its parts. 



The term nurse, applied by Steenstrup to 

 the non-sexual producers, seems inappropriate, 

 and may, in one part at least of his treatise*, 

 have led him into purely speculative compari- 

 sons, such as that with the workers among the 

 gregarious insects ; but in other parts of his 

 essay the author's speculative views are kept 

 in strict subordination to the simple descrip- 

 tion of the facts. Due allowance ought also 

 to be made by the English reader of this work, 

 for the circumstance that, though an excellent 

 translation, it comes to him through the 

 German from its original language. Less ob- 

 jection would probably have been taken to his 

 theory had the term " preparing stocks," or 

 some one conveying a similar meaning, been 

 substituted for that of " nursing individuals." 



Professor Owen also, though admitting in 

 the full extent the peculiar and important 

 nature of the phenomena of alternate genera- 

 tion, objects to the term nurse or nursing 

 animal, as calculated to mislead, and holds the 

 views of Steenstrup, embodied in the phrase 

 "Alternating Generation," to be defective, as 

 not affording any real explanation of the na- 

 ture of the phenomena, or rather, as being no 

 more than a statement of the facts, without 



* On Alternate Generations, p. 112. 



referring them to a sufficiently general law or 

 connecting principle. 



Professor Owen's attention had been, before 

 1843, attracted to the remarkable non-sexual 

 multiplication of the summer Aphides, the 

 structure of which he minutely examined, and 

 had been led to the opinion* that the germs 

 from which the offspring of this non-sexual 

 generation arises are the remains of the ori- 

 ginal germ-substance of the yolk, which have 

 not been applied to the formation of organised 

 textures in the individual immediately deve- 

 loped from tiie ovum. Professor Owen has 

 given the name of Parthenogenesis, or Virgin- 

 production, to this mode of generation, and in 

 the very able and ingenious Essay under that 

 title, published in 184-9, containing the sub- 

 stance of two lectures, introductory to a most 

 instructive course of lectures on the Generation 

 and Development of the Invertebrated Animals, 

 and also in various parts of these lectures-}-, 

 has communicated a more lengthened exposi- 

 tion of his views. " The progeny of the im- 

 pregnated germ-cell," says he, " form the tis- 

 sues, &c., but not all of them are so employed, 

 some of the derivative germ-cells may remain 

 unchanged, and become included in that body 

 which has been composed of their metamor- 

 phosed and diversely combined or confluent 

 brethren : so included, any derivative germ- 

 cell, or the nucleus of such, may commence 

 and rejjeat the same processes of growth by 

 imbibition, and of propagation by spontaneous 

 fission, as those to which itself owed its origin; 

 followed by metamorphoses and combinations 

 of the germ-masses so produced, which concur 

 to the development of another individual ; 

 and this may be, or may not be, like that 

 individual in which the secondary germ-cell 

 or germ-mass was included." He states 

 farther, that, the lower the animal in the scale 

 of life, the number of derivative germ-cells 

 and nuclei which retain their individuality and 

 spermatic power is greater, and the number of 

 those that are metamorphosed into tissues 

 and organs, less. The simplest animals are 

 nothing more than nucleated cells, or in the 

 minute and microscopic ones, as Gregarina and 

 Polygastria, one nucleated cell only ; the mid- 

 dle layer of the wall of the Hydra he describes 

 as consisting of nucleated cells. In Compound 

 Polypes and Parenchymatotis Entozoa a large 

 quantity of derivative germ-cells is retained 

 among their textures or other parts ; and the 

 same is the case at the caudal extremity of 

 the Nais and young Annelida. Professor 

 Owen informs us that he has observed the 

 germs of the viviparous Aphides in the em- 

 bryoes near the simple digestive sac before 

 any organs have been formed for their re- 

 ception, and that when these germs are after- 

 wards included in the tubes which correspond 

 to the ovaries and oviducts, he regards them 

 as comparable to the germ mass in its mi- 



* See Lectures on the Comparative Anatomy of 

 the Invertebrated Animals, IM.'i, p. '234. and 3G6. 



f As published in the Medical Times, vols. xix. 

 ami xx. 



J Oa Parthenogenesis, p. 5. 



D 3 



