The part of the body in which these nourishing substances are supposed to be produced is not specifically stated, but in one passage Bonnet says that the fluids that flow toward the head are there used up in that organ, and we may infer that he held a similar view for the posterior region. He offers no explanation of the cause of the flow of these substances in a given direction, and in this respect his hypothesis lacks support where it is most needed. In fact, it is no more improbable that a head germ should always develop at the anterior end and a tail germ at the posterior end, than that head-forming substances should flow in one direction and tail-forming in another. It is not that it is worth while to object to Bonnet’s hypothesis on the ground that it does not explain everything, but it is worth while to point out that it gives only the appearance of an explanation, and that it begs the whole question by the assumption of particular nourishing fluids flowing in definite directions. So far as the blood is concerned, we know that the different parts of the body take from it those substances or fluids that they make use of, not that special fluids flow to particular regions. It is probable that Bonnet thought of the blood rather than of any other subtler fluids passing through the tissues; and, if so, there is nothing that we know in regard to the behavior of the blood that lends support to Bonnet’s idea. Bonnet takes care to state that the pre-formed germs may not appear to us like miniature copies of the part into which they develop, but they are so constructed that, as they absorb nourishment and become larger, they assume a characteristic form. Weismann, who has also accepted the pre-formation hypothesis to account for the development of the egg, has applied the same conception of pre-formation to the process of regeneration. He believes that partial, latent germs are present in different parts of the body, and that these germs are present especially in animals that are liable to injury and in those parts of the body that are likely to meet with accidents. In these essential respects, Weismann’s idea is the same as Bonnet’s; but in regard to the location of the germs, and their manner of awakening, and as to how the forms, liable to injury, have acquired their power to regenerate, Weismann adopts more modern standards. He believes that the germs are located in the nucleus. Those that bring about the development of the egg are supposed to be different from those that bring about regeneration, because the method of regeneration is generally different from the method of development of the egg.