128 



The Nucleus and Cytoplasm in Development 



tion" has taken place, the cytoplasmic bridge, 

 may be severed so that the two halves of 

 the egg will develop independently. 



This experiment produces pairs of embryos 

 or larvae of various tj^pes (Fig. 24); (1) 

 normal twins, frequently showing slight uni- 

 lateral defects of the eye, balancer, gills, 

 and forelimb bud on the "inner" side facing 

 the partner; (2) one normal and one micro- 

 cephalic or acephalic larva; (3) one normal 

 larva and a "ventral embryo" imable to 

 gastrulate normally or to form any axial 



Fig. 25. Ventral \dew of an older pair of lateral 

 twin larvae obtained by constriction of the egg 

 shortly after fertilization, a, The "left" larva shows 

 normal position of the aortic trunk and of stomach, 

 liver (stippled) and gall bladder, b, The "right" 

 larva exhibits complete situs inversus of the heart 

 and viscera. 



organs. These types of defective embryos 

 correspond to those obtained by constriction 

 of cleavage, blastula and early gastrula 

 stages (Spemann, '01, '02; Spemann and 

 Falkenberg, '19). Their origin may be ex- 

 plained by assuming that the plane of con- 

 striction, which always includes the animal- 

 vegetal axis or runs parallel to it, may form 

 any angle with the invisible plane of bi- 

 lateral symmetry of the egg. This plane in 

 turn is determined by an early localization of 

 the area of the future dorsal lip of the 

 blastopore, i.e., of the center of organiza- 

 tion, which is instrumental in the establish- 

 ment of the embryonic axis. The fate of each 

 egg fragment would depend on what share 

 of this important region it obtains (Fig. 24). 

 Recent experiments of Dollander ('50) on 

 newt eggs with visible symmetry produced 

 more variable results, including the forma- 

 tion of a complete embryo from a ventral 

 blastomere at the two-cell stage, under cer- 

 tain conditions. 



It should be noted that it has not been 

 possible as yet to demonstrate any progress 

 in the localization of cytoplasmic factors of 



development in the salamander egg between 

 the time of fertilization and the beginning 

 of the first cleavage. A series of constrictions 

 performed from 10 to 50 minutes after in- 

 semination, before completion of the second 

 maturation division, gave essentially the 

 same results as a second series in which the 

 eggs were divided between three and four 

 hours from insemination. The future center 

 of organization must be localized on the 

 dorsal side of the egg shortly after fertiliza- 

 tion. Since the gray crescent, which marks 

 this area in the eggs of other species of 

 amphibians, does not appear until about one 

 and one-half hours from fertilization, this 

 redistribution of the superficial pigment is 

 no more than a secondary expression of a 

 more fundamental though invisible organiza- 

 tion of the egg which may date back to 

 prefertilization stages (Fankhauser, '30, '48). 



In addition to a definite differentiation 

 between the dorsal and ventral sides of the 

 egg, the newt egg also shows signs of an 

 early establishment of the bilateral asym- 

 metry of the vertebrate body. The "left" 

 twin of a pair as shown in Figure 24a always 

 has the normal situs of the heart and viscera 

 (Fig. 25«), while about 50 per cent of the 

 "rirrht" twins exhibit situs inversus (Fig. 

 25^). Again, constriction of the egg shortly 

 after fertilization and at various later stages 

 up to the early gastrula gives the same re- 

 sults. The possible basis of the early origin 

 of this asymmetry has been discussed in 

 detail bv Spemann and Falkenberg ('19) and 

 should be investigated further. 



Relation of Initial Mass of Cytoplasm to 

 Final Rody Size. The relations between egg 

 size and body size have been studied in a 

 number of species which include large and 

 small races. Generally there is no correla- 

 tion between the two; different breeds of 

 rabbits have eggs of the same size (Painter, 

 '28; Castle and Gregory, '29; Gregory and 

 Castle, '31); in selected large and small 

 lines of Drosophila funebris, the size of the 

 eggs varies independently of the body size; 

 i.e., genetically small races may have large 

 eggs (Zarapkin, '34). 



Experimental reduction of the size of the 

 egg through fragmentation at an early stas^e 

 of development produces proportionately 

 smaller embryos and young larvae, made up 

 of a reduced nvamber of cells of normal size. 

 However, if such dwarf larvae from iso- 

 lated egg fragments or blastomeres of a 

 newt are allowed to develop in the presence 

 of an ample supply of food, they will attain 

 approximately normal size before metamor- 



