. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine . ria? round female pronucleus, asseen in the living egg (E. F, H, and I from picric acid preparations); L, expulsionof the first polar cell. (Haddon.) then, are simply expelled ; they take no part in the developmentof the ovum ; and their extrusion is to be regarded as a prepar-ation for the progress of the cell, whether this event follows orprecedes the entrance of the male cell into the ovum. It is wor-thy of note that the ovum may become amceboid in the regionfrom which the polar glo
. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine . ria? round female pronucleus, asseen in the living egg (E. F, H, and I from picric acid preparations); L, expulsionof the first polar cell. (Haddon.) then, are simply expelled ; they take no part in the developmentof the ovum ; and their extrusion is to be regarded as a prepar-ation for the progress of the cell, whether this event follows orprecedes the entrance of the male cell into the ovum. It is wor-thy of note that the ovum may become amceboid in the regionfrom which the polar globules are expelled. The remainder of the nucleus(female pronucleus) now passesinward to undergo further changes of undoubted importance,possibly those by virtue of which all the subsequent evolutionof the ovum is determined. This brings us to the considerationof another cell destined to play a brief but important role on thebiological stage. REPRODUCTION. 61 THE MALE CELL (SPERMATOZOON). This cell, almost without exception, consists of a nucleus(head) and vibratile cilium. However, as indicating that the. Fig. 60.—Spermatozoa (after Haddon). Not drawn to scale. 1, sponge; 2. hydroid;3, nematode: 4, cray-fish; 5, snail; 6, electric ray; 7, salamander; S. horse; 9, many spermatozoa, as in Nos. 7 and 9, an extremely delicate vibratile band ispresent. latter is not essential, spermatozoa without such an appendagedo occur. The obvious purpose of the cilium is to convey themale cell to the ovum through a fluid medium—either the waterin which the ova are discharged in the case of most invertebrates,or through the fluids that overspread the surfaces of the femalegenerative organs. The Origin of the Spermatozoon.— The structures devoted tothe production of male cells (testes), when reduced to their es-sentials, consist of tubules, of great length in mammals, lined 62 COMPARATIVE PHYSIOLOGY. with nucleated epithelial cells, from which, by a series ofchanges figured above, a
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