(3) The origin of organs. The origin of the primitive organs is intimately connected with that of the germinal layers. The first differentiation of the segmented ovum results in the cells of the embryo becoming arranged as two layers, an outer one known as the epiblast and an inner one as the hypoblast. The outer of these forms a primitive sensory organ, and the inner a primitive digestive organ.
(4) The gradual evolution of the more complicated organs and systems of organs.
This part of the subject, even more than that dealing with questions of Phylogeny, is intimately bound up with Comparative Anatomy; without which indeed it becomes quite meaningless.
A study of reproduction logically precedes that of Embryology. Reproduction essentially consists in the separation of a portion of an organism which has the capacity of developing into a form similar to that which gave it origin. The simplest modes of reproduction are those which occur amongst the Protozoa.
In this group, reproduction may take place in a great variety of ways. These may be classified in three groups: (1) fission, (2) budding or gemmation, (3) spore formation.
Reproduction in all these ways may take place either subsequently to and apparently in consequence of a very important process known as conjugation, which consists in the temporary or permanent fusion of two or more individuals, or spontaneously, i.e. independently of any such previous conjugation.
Reproduction by fission consists simply in the division of the organism into two similar parts, the nucleus when present becoming divided simultaneously with the cell body. This mode of reproduction is the simplest conceivable, and is not followed by a development, since the two organisms produced are exactly similar, except in size, to the parent form. Besides single fission, a process of multiple fission may take place, as amongst the Flagellata, where Drysdale and Dallinger have shewn that an individual enclosed within a structureless cyst may divide first into two, then into four, and so on.
The process of budding differs mainly from that of simple fission in the fact that the two organisms produced are dissimilar in size, and also that the separation of the smaller organism from the larger is preceded by a process of growth in the latter, so that in the separation of the bud no essential part of the parent form is removed. This mode of reproduction is found amongst the Infusoria, Acineta, &c. An interesting variation in it is the internal gemmation of many of the Acineta, where a portion of the internal protoplasm with part of the nucleus is separated off to form a fresh individual. This mode of gemmation is connected by a series of gradations with the normal external gemmation. The organisms produced by gemmation are not always similar at birth to the parent; e.g. Acineta.
Both fission and gemmation when incomplete lead to the formation of colonies.