[Partial Protection of the Ovum.]—There are some exceptions, however, to this general rule among fishes and reptiles. Even fishes manifest a degree of parental solicitude in certain cases. The male of a species of South American fish gathers up the eggs after fecundation has taken place, and carries them in his mouth until they are hatched. Another male fish carries the eggs of his mate in a little pouch upon the lower and posterior part of his body.
Certain species of frogs carry their eggs wound about their legs; others suspend them from the abdomen. Another variety carries its young upon its back. Prof. Wyman describes a "swamp toad" which patiently takes the eggs of his mate, one by one, and fastens them upon her back, observing great regularity in arrangement. These several devices are evidently for the purpose of protecting, in some degree, the young individual during the helpless stage of its existence.
[Development in the Higher Animals and Man.]—Higher animals are less prolific, and their development is a more complicated process; hence, their young need greater protection, and, for this reason, the ova, instead of being discharged from the body of the female after fecundation, are retained.[3] As we have seen that a suitable receptacle is sometimes provided outside of the body, so now a receptacle is needed, and is provided in the interior of the body of the female. This receptacle is called
3 Curious examples of internal development sometimes occur in animals which usually deposit eggs. Snakes have been known to produce both eggs and living young at the same time. At the annual meeting of the American Society for the Advancement of Science, at Detroit, Mich., in August, 1875, we had the pleasure of examining a specimen, exhibited by Prof. Wilder, of a chick which had undergone a considerable degree of development within the ovary of the hen. It had a head, a rudimentary brain, and internal viscera, but no feathers nor limbs. It was, in fact, an egg hatched before it had been laid. The anomaly excited much interest at that time and since among biologists.
[The Uterus.]—This is a hollow, pear-shaped organ, located in the median line, just behind the bladder, between it and the rectum. It is supported in place by various ligaments and by the juxtaposition of other organs. Its larger end is directed upward, and communicates upon each side with a very narrow tube which is prolonged outward on either side until it nearly touches the ovary of the same side. Its lower and smaller end fills the internal extremity of the passage previously described as the vagina. When an ovum is matured, it escapes from the ovary into the narrow tube referred to, called the Fallopian tube, and passes down into the cavity of the uterus. If fecundation does not occur, it is expelled or absorbed after six to twelve or fourteen days. If copulation occurs, however, zoösperms are brought into the cavity of the uterus, and, coming in contact with the ovum, fecundate it. This is conception. When the natural process is allowed to proceed, development occurs.
[Uterine Gestation.]—This is the term applied to the process last referred to. We shall not attempt to describe in detail this most wonderful and intricate of all living processes; but will sketch only the chief points, leaving the reader who would obtain a more complete knowledge of the subject to consult any one of the numerous physiological and obstetrical works which deal with it in a very exhaustive manner.
As soon as the ovum is impregnated by the male element, it begins a process of symmetrical division. The first division produces two cells out of the single one which first existed. By the next division, four segments are produced; then eight, sixteen, etc. While this process is going on, the ovum becomes adherent to the internal wall of the uterus, and is soon enveloped by its mucous membrane, which grows up about and incloses it.
[The Primitive Trace.]—When the process of segmentation has advanced to a certain point, the cells are aggregated together in a compact layer at the surface. Soon a straight line appears upon this layer, which is called the primitive trace. This delicate line becomes the basis for the spinal column; and upon and about it the whole individual is developed by an intricate process of folding, dividing, and reduplication of the layer of cells. One end of the line becomes the head, and the other becomes the tail. Even man has a caudal appendage at an early stage of his existence. After a further lapse of time, little excrescences, buds, or "pads," appear in the proper positions to represent the arms and legs. After further development the ends split up into fingers and toes, and by the continued development of the parts, perfect arms and legs are formed.
[Curious Relation to Lower Animals.]—It is a very remarkable fact that in the lower animals we have numerous examples in which the permanent condition of the individual is the same as some one of the stages through which man passes in the process of development. The same author previously quoted makes the following interesting statements:—