GROWTH, REGENERATION AND REPRODUCTION 1121 



animals and plants. Moreover, stimulated by the close similarity 

 between the changes presented by the developing ova of widely differ- 

 ent species, Hackel formulated his Gastraea-theory which states that 

 all forms of blastoderms, consisting of two germinal layers, may be 

 regarded as a modified simple gastrula. In the same way as the gas- 

 trula is the beginning of the formation of a single individual, so may 

 an animal of similar simple construction be considered as the ancestor 

 of all multicellular forms. While this view has been widely dissemi- 

 nated, it lacks confirmation, because it has not been proved that a 

 gastrula gives rise to any other entity than that from which it has 

 arisen. Furthermore, the preceding discussion pertaining to the ferti- 

 lization of the ovum must have shown that the differentiation really 

 takes place much sooner, i.e., at the time of fusion of the pronuclei. 



The Darwinian theory of evolution is based upon slowly developing 

 anatomical peculiarities to which have been added certain data 

 derived from artificial selection. Thus, an experimental element was 

 introduced for the first time which, however, was again lost sight of 

 later on. Opposed to this contention is the theory of mutation which 

 is founded upon phenomena of cell-life. 1 Since racial characteristics 

 are no doubt mapped out in the segmenting ovum, all homologies or 

 similarities appearing later on, must find their origin in the material 

 substance of the fertilized egg. Quite similarly, any modification in 

 the germinal arrangement must give rise to mutations which charac- 

 terize evolution. 



In sexual reproduction it may be surmised that the carriers of the 

 characteristics are the chromosomes, which thus impart to the new 

 individual the peculiarities of its parents. This transfer, however, 

 is not always effected in a proportional measure, but often favors 

 more particularly the male or female parent. We have seen that the 

 mitosis then occurring, is associated with a reduction of the number of 

 chromosomes, and hence, any variation shown by the offsprings may 

 be referred to the qualitative differences in the chromosomes which 

 have been formed during the development of the ovum. The manner 

 in which gross as well as minor characteristics may be transmitted has 

 been more fully illustrated by the results of an elaborate series of 

 experiments performed by Mendel 2 upon different varieties of peas. 

 The bearing of these experiments, however, has not been fully appre- 

 ciated until about the year 1900. At this time DeVries found that 

 the seeds of Lamark's primrose, sown in his experimental garden, gave 

 rise not only to a small percentage of the same type but also to new 

 types of which he recognized seven. When self-fertilized, these muta- 

 tions not always bred true to their type, but produced at times new 

 varieties. Mendel's first experiments were carried out upon peas. 



1 Buff on, Historie naturalle, 1755; Lamarck, Rech. sur I'origination des corps 

 vivants, 1802; St. Hilaire, Princ. de philosophic zoologique, 1830; Weissman, 

 On germinal selection as a source of definite variation, 1896, and DeVries, Ernahr- 

 ung und Zuchtwahl, Biol. Zentralbl., xx, 1900. 



2 Versuche iiber Pflanzenhybriden, Briinn, 1866. 



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