The Role of Nucleus and Cytoplasm 



141 



ognized individually by differences in size 

 and shape, it is possible to analyze the ab- 

 normal chromosome complements in detail 

 (Fankhauser, '34) and to demonstrate di- 

 rectly their random composition and ex- 

 treme unbalance. As would be expected, all 

 of these eggs die in the blastula stage. 



Recently Morgenthaler ('48) transplanted 

 pieces from androgenetic Tritiu-us blastulae, 

 of constitution presvimably similar to that 

 shown in Fig. 35d, to the flank of normal 

 neurulae. Sections of the host embryos fixed 

 in an early tail-bud stage showed the pres- 

 ence of mitotic figures in the graft, some of 

 which contained fewer than twelve chromo- 

 somes. This indicates that subhaploid cells 

 may continue to divide and survive for some 

 time in the normal environment of a diploid 

 embryo. 



QUALITATIVE CHANGES: EXPERIMENTS 



INVOLVING HYBRIDIZATION BETWEEN 



TWO SPECIES 



The introduction of a nucleus of one spe- 

 cies into the egg cytoplasm of another offers 

 an opportunity to study the effects of the 

 combination of two components that presum- 

 ably differ in a qualitative way. A variety 

 of unusual modifications of the processes of 

 development are produced that have been 

 analyzed most intensively in echinoderms 

 and amphibians. 



DIPLOID HYBRIDS 



Echinoderms. The development of most 

 echinoderm hybrids has not been followed 

 beyond the pluteus stage. The internal phe- 

 nomena of fertilization may be completely 

 normal, as in the cross Echinus esculentus 2 

 X Echinus (Psammechinus) miliaris $ . In 

 the reciprocal cross a small percentage of 

 the eggs develop and a large proportion of 

 these show elimination of one or a few chro- 

 mosomes in the first cleavage mitosis (Don- 

 caster and Gray, '13). In the combination 

 Paracentrotus (Strongylocentrotus) lividus 9 

 (N = 18) X Sphaer echinus granulans $ 

 (N = 20) about sixteen chromosomes lag in 

 the equator of the spindle during the first 

 cleavage mitosis. The paternal origin of these 

 chromosomes was demonstrated by isolation 

 of a Sphaerechinus sperm nucleus in a frag- 

 ment of a Paracentrotus egg; for at the late 

 anaphase of the first cleavage mitosis about 

 four normal chromosomes could be counted 

 at each pole, while the remaining chromo- 

 somes were left behind in the spindle. In all 

 crosses with sperm from Arbacia (N = 20), 



chromosome behavior during early cleav- 

 ages was normal; but a sudden crisis oc- 

 curred at the blastula or gastrula stage when 

 about half of the chromosomes, presumably 

 the paternal ones, were eliminated in each 

 cell and the embryos became abnormal 

 (Baltzer, '10). The incompatibility between 

 most or all of the sperm chromosomes and 

 the egg cytoplasm may thus be expressed in 

 visible disturbances of mitosis in early or 

 late cleavage stages, and development may 

 stop or become abnormal at gastrulation. Al- 

 though hybrid plutei have been obtained 

 from reciprocal crosses between species that 

 differ in the characteristics of the larval 

 skeleton in order to study the possible influ- 

 ence of the maternal cytoplasm, the inter- 

 pretation of the observations is difficult be- 

 cause of the variability of the appearance of 

 the plutei in different series of experiments 

 and the possible influence of external fac- 

 tors such as temperature (cf. Morgan, '27, 

 Chapter 25; P. Hertwig, '36). 



Eggs of sea urchins may be activated by 

 sperm of mollusks (Mytilus) and produce 

 typically maternal plutei. Development is 

 gynogenetic since the sperm head is resorbed 

 withovit forming a sperm nucleus while the 

 sperm centrosome apparently functions as the 

 division center (Kupelwieser, '09). 



Amphibians. A wealth of interesting ob- 

 servations has accumulated on the results of 

 hybridization between various species of 

 frogs, toads, and salamanders (cf. the review 

 by P. Hertwig, '36, and the more recent 

 papers by J. A. Moore, '41-48, Blair, '41, and 

 Baltzer, '52). The following lines of investi- 

 gation are of particular importance for the 

 analysis of our problem: 



Cytology of fertilization in relation to de- 

 velopment of hybrid eggs (Bataillon and 

 Tchou-Su, '29; Tchou-Su, '31). Figure 36 

 illustrates the variety of fertilization phe- 

 nomena, which range from mere activation 

 of the egg without penetration by the sperm, 

 to completely normal fertilization. In the 

 latter case, development may come to an end 

 at the beginning of gastrulation, sometimes 

 with visible disturbances of mitosis in the 

 blastula, as in the cross T. palmatus $ X 

 Salamandra maculosa $ (Schoenmann, '38), 

 or normal hybrid larvae and adults may be 

 produced. 



Analysis of early arrest of development. 

 The factors involved in the arrest of de- 

 velopment at gastrulation in many crosses 

 have been analyzed in different types of ex- 

 periments. Transplantation of pieces from 

 hybrid blastulae (T, palmatus 2 X S. ma- 



