The Role of Nucleus and Cytoplasm 



127 



chromosomes ("diploid merogony"). If the 

 resulting embryo or larva shows typical ab- 

 normalities or defects, it may be concluded 

 that certain cytoplasmic materials necessary 

 for normal development were lacking in this 

 particvilar fragment; this implies the more 

 general conclusion that, even at fertilization. 



Among vertebrates, so far, the eggs of 

 various species of newts alone have lent 

 themselves to fragmentation before the first 

 cleavage, by means of constriction with a 

 loop of fine hair (Spemann, '14, '28). In 

 these eggs it is possible, furthermore, to have 

 both halves of one egg develop with the 



Fig. 24. Diagram to illustrate the origin of different types of twin embryos following constriction of the 

 unsegmented egg in different planes relative to the median plane. 



a. Constriction in the median plane, dividing the area of the future dorsal lip evenly. "Left" and "right" 

 twin larvae are produced showing slight deficiencies in the development of eye, balancer, gills and forelimb 

 bud on the "inner" side, facing the partner. 



b. Constriction in an oblique plane dividing the dorsal lip area unequally. The left half, with a small 

 lateral portion of the "organization center," develops into a microcephalic larva. The co-twin is normal. 



c. Constriction in the frontal plane isolating the dorsal and ventral halves of the egg. The ventral half is 

 unable to form axial organs but may survive for several days. The dorsal half develops normally. 



certain areas of the egg cytoplasm differ from 

 others in their organization. 



The classic experiments on eggs of various 

 marine invertebrates (see Section VI, Chapter 

 3) demonstrated that in many species factors 

 necessary for the formation of embryonic or 

 larval structures are localized early. In some 

 cases, particularly in Cerebratulus, this loc- 

 alization was shown to progress during mat- 

 uration and fertilization of the egg, by the 

 steadily increasing percentage of defective 

 larvae produced by dividing the egg at suc- 

 cessively later stages. 



normal, diploid complement of chromosomes, 

 by constricting the egg at first partially into 

 a dumbbell shape. Cleavage begins in the 

 half that contains the egg as well as a sperm 

 nucleus. However, following two or more 

 cleavage divisions (depending on the thick- 

 ness of the bridge connecting the two parts 

 of the egg)^ the nucleus located nearest the 

 bridge enters it. During telophase of the 

 following mitosis, one of the daughter nu- 

 clei moves into the hitherto non-nucleated 

 part and initiates a delayed but normal 

 cleavage. Soon after this "delayed nuclea- 



