The works of Francis Maitland Balfour, Volume 2 (of 4)

by Francis M. Balfour

Although there is no doubt that the spermatozoon in most instances plays as important a part as the ovum in influencing the characters of the organism which is evolved from the coalesced product of the ovum and spermatozoon, yet the actual form of the spermatozoon has not, like the form of the ovum, a secondary influence on the early phases of development. A comparative history of the spermatozoon is therefore of less importance for my purpose than that of the ovum; and I shall confine myself to a few remarks on its general structure, and mode of growth. The primary origin of the male germinal cells, and their relation to the sperm-forming cells, is dealt with in the second part of the treatise.

Although the minute size of most spermatozoa places great difficulties in the way of a satisfactory investigation of them, yet there can be but little doubt that they always have the value of cells. In the vast majority of instances the spermatic cell or spermatozoon is composed of (1) a spherical or oval portion known as the head, formed of a nucleus enveloped in an extremely delicate layer of protoplasm, and (2) of a motile protoplasmic flagellum known as the tail; which together with the investing layer of the head forms the body of the cell.

As might be anticipated, the proportion, size, and relations of the parts of the spermatozoon are subject to great variations. The head is often extremely elongated; and it is in many cases rather on theoretical grounds, than as a result of actual observation, that a protoplasmic layer is stated to be continued round the nucleus which forms the main constituent of the head. In some of the elongated forms of spermatozoa, e.g. in Insecta, there is no marked distinction, except in the character of the protoplasm, between the head and the tail. A connecting element is frequently interposed between the head and tail, which appears however to be constituted of the same material as the tail, and sometimes forms a thickening on the tail close below the head (Amphioxus). A very remarkable modification of the tail is found in many Amphibia, Reptilia and Mammalia. In these types there is attached to what appears to be a normal tail a delicate membrane, the outer edge of which is thickened to form a kind of secondary filament. In the living spermatozoon this filament is in a state of constant movement. The membrane winds spirally round the tail.

In the majority of forms the tail of the living spermatozoon exhibits sinuous cilia-like movements. In two groups the movements are however of an amœboid character. These groups are the Nematoda and the Crustacea; and the spermatozoa in both of them frequently present very abnormal forms. In Nematoda they are pear-shaped, cylindrical, spine-shaped, etc., and are mainly formed of protoplasm with a highly refracting nucleus. In the Crustacea the variations of form are still greater. In the Malacostraca they are sometimes simply spherical (Squilla), while in Astacus and a large number of Decapoda they are composed of a nucleated body with stellate rays. In Paludina amongst the Mollusca there are two forms of completely developed spermatozoa existing side by side in the same individual.

The spermatozoa are formed by the breaking up of the male germinal cells, or of cells secondarily derived from them by division. The cells which directly give rise by division to the spermatozoa may be called spermospores and are equivalent to the ova or oospores.

Amongst the Sponges (Halisarca, Schultze, No. [141]) a germinal cell, similar to that which in the female becomes an ovum, repeatedly divides and eventually gives rise to a ball of cells (a spermosphere or sperm-morula), each constituent cell of which becomes converted into a spermatozoon, and may be designated by the special term ‘spermoblast’.

In most Hydrozoa the subepithelial epiblastic cells become converted into germinal cells (spermospores), and then break up to form spermoblasts, each of which becomes a spermatozoon.

In most higher Metazoa the spermospores usually form the epithelium of an ampulla or tube, though more rarely (many Chætopoda, Gephyrea, etc.) they may be derived from cells lining the body cavity, as in the case of ova. The spermatozoa are formed either by the direct division of the spermospores into a number of cells, spermoblasts, each of which grows into a spermatozoon; or by the nucleus of the spermospore becoming subdivided within the cell body, the latter differentiating itself into the tails of the spermatozoa while the segments of the nucleus give rise to the main part of the heads.

In many instances interstitial cells which do not give rise to spermatozoa, are intermingled with the spermospores.

In a good many cases, as first pointed out by Blomfield[26], the whole of each spermospore does not become converted into spermatozoa, but part, either with or without a segment of the original nucleus, remains passive, and carrying as it does the off-budded spermoblasts may be called the ‘sperm-blastophor.’ This passive portion of protoplasm is not employed in the regeneration of the spermoblast. This very singular phenomenon has been observed in Elasmobranchii, the Frog, the Earthworm, Helix, etc.[27], and probably has a much wider extension. In Elasmobranchii (Semper) the passive portions of protoplasm are nucleated, and are placed on the outer side of the columnar spermospores which line the testicular ampullæ; they are not distinctly differentiated till the nuclei, segmented from the nucleus of the primitive spermospore to form the heads of the spermatozoa, have become fairly numerous. In the Frog the passive blastophor also occurs as a nucleated mass of protoplasm on the outer side of the spermospore. In the Earthworm the blastophor forms a central non-nucleated portion of the spermospore; and the whole periphery of each spermospore becomes converted into spermoblasts.