226 THE PHYSIOLOGY OF REPRODUCTION 



by fission or budding, which is usually considered the oldest form 

 of reproduction. In some worms, such as Planaria velata, Ctenodrttus 

 monostyhis, 1 this seems to be the sole means of reproduction which 

 these animals possess. Secondly, we have reproduction by means of 

 gametes, and their fusion to form the zygote or fertilised egg. These 

 two modes of reproduction are essentially similar in many respects, 

 and one has probably been derived from the other in the course of 

 evolution. In many Planarians, any part of the body may give rise 

 to a new individual, provided this portion of the body becomes 

 isolated to a certain extent, or entirely separated from the parent 

 body. Thus Planaria velata, at the end of the season, breaks up 

 into a large number of minute fragments, which contract into round 

 spherical masses not unlike ova in appearance, and the following 

 year give rise to new worms. The gametes or germ-cells are 

 comparable, in Child's opinion, to these fragments of Planaria vein In, 

 and are therefore physiologically old cellsj differing in no way from 

 other cells of the body, except that they require conjugation with 

 one another, in order to undergo reorganisation and rejuvenation, 

 but still in many instances they are able to undergo this process in 

 the absence of conjugation or fertilisation, as in parthenogenetic 

 development. Child arrives at this conclusion as the result of a 

 long series of investigations into the metabolic growth rate of 

 regenerating worm fragments, and the investigation of the metabolic 

 rate at different periods of the animal life cycle. To determine this 

 rate certain tests are employed. In an aqueous solution of potassium 

 cyanide or weak alcohol, in which death of the worm fragment or 

 egg-cell occurs in from a few minutes to several hours, the sus- 

 ceptibility varies with the metabolic rate ; thus in a fragment in 

 which the metabolic rate is high, as shown by its consumption of 

 oxygen, or output of C0 2 , or its functional activity, the susceptibility 

 is also very great, and all conditions which increase metabolic 

 activity increase susceptibility. If, however, the narcotics are used 

 in such low concentration as to admit of partial, but not complete 

 tolerance to the new conditions, then those fragments of the worm 

 in which growth is most rapid and the metabolic rate is highest 

 are the last to be killed, as they have the greatest power of 

 adjustment to the action of the narcotic. This method is the 

 reverse of the first, and may be called the indirect susceptibility 

 test. By the application of these tests and others of a similar 

 character, in which potassium permanganate or phenylurethane 

 are used, Child has shown that the growing organism possesses 

 definite axial growth gradients, in which the metabolic rate is 



1 Monticelli has described a sexual phase in this animal. Atti del Congresso 

 dei Naturalisti italiani, 1906, Milan, 1907. 



