40 GENERAL CONCEPTIONS. 



germ-cells in the germinal epithelium are commonly known by the name of the 

 primitive ova. The transformation of these cells into true ova has been traced 

 in a great many forms, so that the transformation may be considered as de- 

 monstrated conclusively for all vertebrate animals. It is further commonly 

 assumed that the germ-cells or primitive ova also give rise to the male elements, 

 playing in the formation of the testes a role similar to that which they play in 

 the ovary. There is, unfortunately, up to the present time, no conclusive proof 

 by direct observation that the primitive ova are the actual parents of the cells 

 which give rise to the spermatozoa. 



When a germ-cell is transformed into an ovum, it undergoes great enlarge- 

 ment, its nucleus is modified, the protoplasm is changed in appearance and be- 

 comes loaded with yolk granules, and over the surface of the cell appear two 

 membranes, an inner very thin one, called the vitelline membrane, and an outer 

 much thicker one, known as the zona pellucida. (For a fuller description see 

 page 33.) We thus learn that the germ-cells preserve their resemblance to 

 segmentation spheres only during embryonic life. When they become ova, 

 they pass through a series of important changes in their organization. If it is 

 true that these germ-cells also give rise to the male elements, then we must 

 further say that in order to produce those elements the germ-cells pass through 

 another series of profound changes. 



It is further known that in order to evolve the sexual elements, both male 

 and female, the cell which is to produce them divides twice, and in a special 

 manner, which we designate by the term " reduction division." This process is 

 described in all the recent text-books of cytology and histology. It does not 

 fall within the scope of this work, which deals with embryology in the strict sense 

 only. 



The Theory of Heredity. 



We owe to Moritz Nussbaum the theory of germinal continuity — the only 

 theory of heredity which seems tenable at the present time. According to this 

 theory, the germ-cells are set aside during the segmentation of the ovum and 

 preserve the essentially undifferentiated qualities of the protoplasm and nucleus 

 of the ovum, from the division of which they arise. Just as the cells formed 

 during segmentation are capable of producing the various tissues of the body, so 

 the germ-cells have and preserve this faculty. If we term the material of the 

 original ovum germ-plasm, we may say that this germ-plasm gives rise to the 

 various tissue-forming cells which make up the body. And by this very con- 

 version into tissue cells, that germ-plasm is changed, and is no longer, as we have 

 learned before, capable of the full range of development. The germ-cells, on the 



