442 OF GENERATION AND DEVELOPMENT. 



into use at its termination. Thus the whole life of the Insect, up to its 

 last change, may be regarded as one of prolonged embryonic develop- 

 ment ; and the same may be said of that of the Frog, up to the time 

 when its permanent organs are fully evolved. No such ostensible me- 

 tamorphosis takes place, however, in any of the animals which are pro- 

 vided with a "food-yolk;" for this supplies that material for the con- 

 tinued development of the embryo within the egg, which is elsewhere 

 to be obtained out of it ; and thus the embryo is supported, until it has 

 nearly attained its adult condition, although far from having acquired 

 its adult size. Now in all these cases, it is very interesting to remark 

 that the first nisus is towards an extension of the embryonic mass as a 

 membranous expansion (evidently analogous to the cotyledon of the 

 Flowering Plants, 781) over the " food-yolk ;" in this " germinal mem- 

 brane," which forms a sort of temporary stomach, blood-vessels are de- 

 veloped, which absorb the prepared nutriment and convey it to the per- 

 manent portion of the embryonic structure ; and when its function is 

 completed, the store of aliment being exhausted, and the proper nutrient 

 apparatus of the embryo being ready for action, we lose sight of it al- 

 together. We shall find that a similar germinal membrane is formed in 

 the Human ovum, although there is no " food-yolk ;" its formation being 

 apparently requisite for ulterior purposes, and the portion of the mul- 

 berry mass which gives origin to the permanent part of the embryonic 

 structure being comparatively small. 



2. Action of the Male. 



786. The share in the Reproductive Function, which belongs to the 

 Male Sex, essentially consists in the formation and liberation of the 

 fertilizing bodies termed Spermatozoa. These are prepared within 

 peculiar cells, as already described ( 241); and the "sperm cells" are 

 either scattered through the soft parenchyma of the body, as happens 

 among some of the lowest anim'als ; or they are confined to certain parts 

 of it, as in those a little more elevated in the scale ; or they are formed 

 within follicles or tubes, clustered together into an organ of a glandular 

 character, known as the Testis. Such an organ is found in all Insects 

 and Mollusca ; as well as in Vertebrated Animals. In the first of these 

 classes, it is formed on the general plan of their proper glands ( 720) ; 

 being usually composed of tubes, more or less elongated, and some- 

 times terminating in enlarged follicles. In the Molluscs, on the other 

 hand, it is almost invariably composed of clusters of follicles. In either 

 case, the seminal cells are developed within the tubes or follicles, as 

 are the ordinary secreting cells of the Liver or Kidney within the 

 tubes or follicles of those glands ; and their contents are discharged by 

 an excretory duct, which terminates in an organ that conveys them out 

 of the body, either emitting them into the surrounding water (as hap- 

 pens with many Mollusca), or depositing them within the body of the 

 female. It is curious that, in some of the lowest Fishes, we should 

 return to one of the simplest conditions of this organ, a mass of 

 vesicles, without any excretory duct. In these cases the secretion 

 formed within the vesicles escapes, by their rupture, into the abdominal 



