228 THE PHYSIOLOGY OF REPRODUCTION 



stage of senescence, in which the organism slowly grows old. Its 

 functions come to an end through the gradual accumulation of 

 metaplastic substances in the cytoplasm of its cells which finally 

 bring about death. The cycle is restarted by the production of 

 buds or germ-cells by the old organism which are capable of 

 rejuvenescence, so the repetition of the cycle is rendered possible. 

 It results from this conception of the life cycle that no special 

 weight need be given to the many peculiar features connected with 

 the origin and supposed segregation of the germ- cells, to which 

 the Weismann theory of the germ-plasm has attached such great 

 importance. Child has shown that in the Cestode Moniezia, under 

 certain conditions, the spermatagonia can be differentiated from 

 somatic muscle-cells, and that a germ-plasm with a given specific 

 constitution does not hold in the case of this animal. Many of the 

 facts of experimental embryology and regeneration, moreover, clearly 

 render the old theory of the germ-plasm untenable. It is, however, 

 in the explanation of those peculiar types of developiiient in which 

 we find poecilogonie, psedogenesis, and dissogonie taking place that 

 Child's theory gives us such assistance ; under no other theory can 

 these conditions be reduced to any semblance of order or meaning.^ 



In a recent paper ' Stockard ^ has brought forward many 

 interesting facts that throw new light on the question of the 

 development of specific form in animals. Thus by arresting the 

 development of fish embryos at various stages of growth, by keeping 

 them at low temperatures (temp. 5° C.) for a short time, definite 

 abnormalities, such as twin or triple monsters, could be produced. 

 There was a more or less constant relation between the abnormality 

 produced, and the particular stage at which the embryo had been 

 placed in the cold. An embryo kept in the cold at an early stage 

 would subsequently, on being returned to normal temperature, 

 develop into a twin monster; placed in the cold at a later stage 



' The term Pcecilogonie has been introduced by Giard ("La Pcecilogonie," 

 VI. Gongres Int. d. Zol. a Bern, 1904) to describe that condition where an 

 animal possesses two quite different modes of reproduction, such as that shown 

 by the fly Musca corvina, which in the North of Europe reproduces by means 

 of a large number of eggs, while in the South it is viviparous, both types of 

 development resulting in the same adult. Pcedogenesis is a term first introduced 

 by von Baer {Bull. Acad. Imp. St. Peterslmrg, vol. Ix., 1866) where reproduction 

 is carried on by the larva and not the adult, as for instance in the Ghirbnomus 

 larva, which lays eggs that give rise to a perfectly functionless adult gnat. 

 Dissogonie is a term first used by Chun ("Die Dissogonie, eine neue Form 

 der geschlechtlichen Zeugung," Festschrift, f. Lewckart, 1892), and describes that 

 condition where the animal produces ripe gametes in both the embryonic or 

 larval condition and again in the adult stage, as for example in certain 

 Ctenophores such as Bolina hydatina and Enchans multicomis. 



2 Stockard, " Developmental Rate and Structural Expression : An experi- 

 mental study of twins, 'double monsters ' and single deformities, and the 

 interaction among embryonic organs during their origin and development," 

 Amer. Jonr. Anat., vol. xxviii., 1921. 



