156 



The Nucleus and Cytoplasm in Development 



crosses between large and small races of 

 rabbits (Gregory and Castle, '31). The rate 

 of cleavage of eggs, which is correlated with 

 adult body size, is increased under the in- 

 fluence of paternal genes as early as at the 

 third or fourth cell division. Slightly later 

 effects of genes are known in the early em- 

 bryology of Drosophila (Poulson, '40, '45), in 

 the morphology of sea urchin larvae derived 



much earlier, invisibly preparing a situa- 

 tion which is a prerequisite for later effects. 

 There is evidence for both types of activity, 

 the immediate and the distant. Examples of 

 the latter, in purest form, are presented by 

 the so-called maternal effects, in which genes 

 present in the cells of a female, including 

 her immature, premeiotic egg cells, cause 

 developmental effects in her offspring even 



MUTATIONS 



.ol!flJ!.\FAULTY BONE DEVELOPMENT 



NORMAL STAGES 



W Wl^*^ E M I A 



Uk\uROGENITAL abnormalities, |t20 BIRTH 



^PERFORATE ANUS 



FIDA, CLOACA 



IFAILURE OF NOTOCHORD, SOMITES, 

 , POSTERIOR TRUNK REGION AND 

 ILICAL CIRCULATION 



Ki Ki IDUPLICATIONS, ABNORMALITIES OF 

 ALLANTOIC DERIVATIVES 



FAILURE OF ORGANIZATION 

 AND MESODERM FORMATION 



LURE OF IMPLANTATION 



FAILURE OF PREIMPLAN 

 TATION STAGES 



ESTABLISHMENT OF MATERNAL 



ClRCULATIOh 



8 TURNING OF EMBRYO 



CLOSURE OF NEURAL FOLDS 



SOMITES 



ALLANTOIS 



NOTOCHORD 



7 PRIMITIVE GUT ( TR ANSITOR Y)\E 6G 



^CYLINDER 

 RIMITIVE STREAH 



■6 MESODERM 

 5 IMPLANTATION 

 4 BLASTULA 

 3 MORULA 

 2 CLEAVAGE 



■♦ FERTILIZATION 

 DAYS 



Fig. 40. Some of the lethal mutations affecting embryonic processes in mice (from Dunn, '49). 



from species crosses (Moore, '43), at the 

 morula stage of mice homozygous for cer- 

 tain tailless alleles (Dimn, '49; see Fig. 40), 

 and at the blastocyst stage of mice homo- 

 zygous for the yellow-lethal gene (Robert- 

 son, '42). Figure 40 illustrates other em- 

 bryonic processes in mice affected by lethal 

 mutations. 



While it is obvious in these examples 

 that genie action occurs within a short 

 initial period, namely that preceding the 

 time of an observable effect, it is more 

 difficult to make statements when observable 

 effects occur late in development. Here, genie 

 action may have immediately preceded the 

 observable event or it may have taken place 



though these genes may not have been in- 

 cluded in the zygotic nuclei. This is the well 

 known explanation for the inheritance of 

 dextrality and sinistrality in the snail Lim- 

 naea (Boycott, Diver, and Garstang, '30; 

 Sturtevant, '23). The type of configuration 

 which an individual will attain is determined 

 by a pair of alleles, but not by those present 

 in his own genotype. Rather, the genotype of 

 the maternal tissue in which the oocyte is 

 formed fixes the fate of the next generation. 

 This is accomplished by imparting to the 

 cytoplasm of the egg the property, unknown 

 in its essence, of initiating the sequence of its 

 spiral cleavages in either a clockwise or a 

 counterclockwise direction. 



