[138] 



OVUM. 



some other animals, in which, had it been 

 present, it could scarcely have escaped so 

 careful a scrutiny, warrant the belief that in 

 a certain number of animals the spermatozoa 

 do actually penetrate the apparently entire egg- 

 covering. 



It is not my design to enter here upon the 

 consideration of the mode and nature of the 

 action exerted by the spermatic matter or the 

 spermatozoa in producing the changes of 

 fecundation. Upon this subject the reader 

 may with great advantage and interest consult 

 the latter part of the article Semen in this 

 Cyclopedia by R. Wagner and Leuckart, the 

 papers of the late Mr. Newport in several 

 recent volumes of the Philosophical Trans- 

 actions, and the learned article by Professor 

 Leuckart on Generation contained in the 

 fourth volume of R. Wagner's Handbuch der 

 Physiologic. I will only remark in passing 

 that from Mr. Newport's and other researches 

 it appears that while the actual mixture of an 

 appreciable quantity of the spermatic sub- 

 stance is necessary to induce fecundation, the 

 extreme rapidity with which the action takes 

 place, the minuteness of the quantity of the 

 spermatic matter which is sufficient to induce 

 it, and the fact now observed in a variety of 

 instances that the spermatozoa which have 

 entered the ovum remain apparently little 

 changed for a considerable time after the 

 changes of the ovum consequent on fecunda- 

 tion have made some progress, lead to the 

 conclusion that there is something in the 

 nature of this action inconsistent with the 

 idea that it is one of mere combination in 

 substance of the developed contents of the 

 male and female generative products. But 

 whether this is to be referred to the class of 

 " contact actions " of which themselves so 

 little is known, or to what other kind of 

 action it may be compared, the ascertained 

 facts do not enable us in the least to deter- 

 mine. The almost universal presence of vi- 

 bratory motion in the spermatozoa during the 

 time in which they retain their fecundating 

 power, naturally led physiologists to connect 

 that motion with the fecundating action ; but 

 on the other hand, the occasional, though rare 

 examples in which the spermatozoa are en- 

 tirely motionless, seem sufficient to cause the 

 rejection of the view that the force which 

 produces the vibratory motion is identical 

 with that which calls forth the series of 

 histogenetic and organogenetic changes which 

 result from fecundation. 



But the consideration of this subject would 

 lead us into the discussion of the whole 

 question of vital forces, which in its present 

 unsatisfactory state it is desirable to avoid. 

 The physiologist agrees, for the sake of con- 

 venience of expression, to adopt the terms of 

 power, property, force, &c., to denote the con- 

 ditions necessary for the occurrence of certain 

 actions or changes. He employs the term 

 vital force merely as the indication of the 

 supposed cause or causes of an ascertained 

 regular sequence of vital phenomena ; but all 

 philosophical accuracy rejects the idea of any 

 unseen separate and single force which is at 



work in bringing about the sequence in ques- 

 tion. The fecundating power of the semen 

 is an expression used only for convenience 

 to denote the invariable sequence or relation 

 as cause and effect which has been observed 

 to subsist between the contact of spermatic 

 matter with the ovum, and the changes in the 

 latter which follow on the act of fecundation. 

 We might with as much propriety have given 

 a name to a separate power residing in the 

 egg or its germ which render it susceptible of 

 fecundation, as of a special power belonging 

 to the semen by which that susceptibility 

 of the ovum is acted upon. The efficient 

 cause of the process of fecundation can only 

 be educed, as in all physical as well as vital 

 changes, from a perfect knowledge of all its 

 phenomena, and the statement of the efficient 

 cause of such actions is only the expression 

 of the most general and best known law to 

 which a full acquaintance with the phenomena 

 enables them to be reduced. Fecundation is 

 to be regarded as a purely vital change, seeing 

 that it takes place only in the usual conditions 

 of vitality; but, like all other vital changes, 

 it appears more probable that a variety of 

 conditions of the organic matter rather than 

 any one known property or condition are 

 necessary for its occurrence. 



In endeavouring to deduce the most ge- 

 neral phenomena which accompany this re- 

 markable change, it may be said that fe- 

 cundation consists essentially in the mutual 

 action of two different organised bodies, 

 which are respectively formed from two 

 different cells ; the ovigerm and the sperm- 

 germ. If we may form any general con- 

 clusion from what may be so well observed 

 in Nematoid Worms, the development of the 

 ovum and the spermatic cells from their re- 

 spective germs is remarkably similar, for in 

 both the internal cell is developed from a 

 minute molecule from within, while the ex- 

 ternal part is deposited from without. The 

 spermatozoa are formed in connection with 

 the nucleus or nuclei of the sperm-cell ; and 

 the germinal part of the ovum, though it con- 

 sists mainly of the granular part of the yolk, 

 which is directly formative, very probably 

 comprehends also in some shape or other the 

 effused contents of the germinal vesicle. In 

 this way, then, we may conjecture that in the 

 act of fecundation the products of the original 

 cell-germs meet and combine or mutually 

 influence each other. The cell-germs, then, 

 are the links in the chain of organic connec- 

 tion between either or both the parents and 

 the progeny capable of being developed 

 from the fecundated ovum. Such a view, 

 though still in a great measure speculative, 

 seems to be in accordance with the facts 

 known as to the perfect transmission of the 

 structure and qualities of either or of both 

 parents to the offspring.' 



6. Immediate effects of fecundation on the 

 ovum ; segmentation, and first changes of the 

 ovum related to the commencement of em- 

 bryonic development. 



It does riot come within the scope of the 

 present article to describe in detail the pro- 



