. Analysis of development. Embryology; Embryology. 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 evi
. Analysis of development. Embryology; Embryology. 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 IA 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 actio
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