228 



FUNDAMENTALS OF CYTOLOGY 



upon it by the plant which bore it and that this effect is soon nullified as 

 new cytoplasm is produced under the influence of the hybrid nuclei in the 

 plant into which the fertilized egg grows. 



An example of the brief temporary effect of the maternal cytoplasm 

 upon the offspring is seen in seeds of stocks (Matthiola) (Fig. 167). The 

 epidermis of the embryo is dark blue inM. incanasind yellow inilf . glabra. 

 When the two are crossed, vnt\\ incana as the maternal parent, the 

 hybrid's seeds are dark blue. When the reciprocal cross is made, the 

 seeds vary from clear yellow to light blue. Thus the cytoplasms of 

 the two species react differently to the same heterozygous genes in the 

 hybrids' nuclei: incana cytoplasm develops a blue color strongly and at 

 once, whereas glabra cytoplasm does so weakly or not at all. When the 

 two kmds of hybrid are selfed, they behave alike in yielding blue seeds 

 with varying depth of color and yellow seeds in the ratio of 3:1. 



Fi dark blue 



h, light blue 

 to yel low 



3 blue 



(dark to light) 

 I yellow 



3 blue 



(dark to light) 

 I yellow 



M. INCANA 

 epidermis 

 of embryo 

 dark blue 



M. GLABRA 



epidermis 



of embryo 



ye How 



Fig. 167. — Effect of reciprocal crossing in Matthiola (stocks). Explanation in text. 



{Based on data of C. Correns.) 



The same phenomenon is observed among animals in the case of cer- 

 tain larval characters. For example, in cross-fertilized sea urchin or 

 fish eggs the rate and the type of cleavage are the same as in the mother, 

 no matter what the direction of the cross. They are characters impressed 

 upon the egg during its ovarian history, and the male nucleus fails to 

 change the condition already induced in the egg by maternal nuclear 

 factors. When the hybrid matures and produces eggs, however, it is 

 found that these all show the dominant rate and type of cleavage, no 

 matter which parent contributed the dominant factor. This shows that 

 the male does affect the character in the second generation. Breeding 

 experiments with moths and butterflies have given similar results with 

 respect to some embryonic characters. The conclusion is that certain 

 characters whose differentiation is initiated in the egg cytoplasm before 

 syngamy, although fundamentally Mendelian in their inheritance, may 

 be peculiar in that the visible effect of the male gamete is delayed for one 

 generation. 



