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                                                               APPENDIX C                                                        263

their somatic origin. And if they be not somatic, with the actual facts of development as they are, with the pre­-embryonic appearance of the germ-cells, a direct develop­ment is inconceivable, is as impossible as epigenesis.

On the one hand, there are the three doctrines of somatic origin of germ-cells, direct development, and epigenesis; on the other, a like number, morphological continuity of germ-cells, antithetic alternation of genera­tions, and evolution, or unfolding.* All the known facts of development support the latter trilogy. It is out of question to discuss, on the present occasion, all the bear­ings of these three doctrines, or even to foresee these. Of necessity they are connected together; each is an essential part of the theory of development. Outside embryology and zoology their bearings upon scientific problems will undoubtedly be great, but here it may be instructive to point to one or two little things, which they help to explain in zoology and embryology. The position of the sponges, and their mode of development, have been never-ending themes of discussion with zoologists, while the evolution of the mammalian tropho­blast has not been less fruitful of speculation. For brevity, the facts may be placed in tabular form:

LIFE-CYCLES

Name.

Asexual generation

Sexual generation

Nemertine. 

Pilidium

Nemertean Worm

Sponge

Sponge-larva and adult.

Absent

Sea-urchin

Pluteus

Sea-urchin

Hydra

Hydra

Absent

Hydroid polype  (Campanularia).

Hydroid colony

Medusa

Raja batis

Blastoderm, transient nervous system, etc

Skate

Mammal (e.g., man or rabbit.)

Trophoblast (chorion).

Mammal

 

    * Differs from the older views in this way: The evolution of Bonnet, Haller, and others, was an evolution with preformation, that of Weismann is an evolution with predestination.

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Contents Page

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