1124 REPRODUCTION 



of a normal solution, or about 14-6 per cent.) has been added, those 

 of the eggs which have formed membranes develop into swimming 

 larvas that rise to the surface. These larvas develop into perfect 

 sea-urchin larvas or ' plutei ' as fast as the larvas of eggs fertilized 

 with sperm. It has lately been shown that pricking of the unfer- 

 tilized egg of a frog with a needle suffices to induce normal develop- 

 ment of the egg (Bataillon). These parthenogenetic frogs have been 

 successfully reared, and apparently are all males (Loeb). These 

 observations have an important bearing on the question of the deter- 

 mination of sex. In the frog it would seem that the eggs are all 

 alike, since in the absence of a spermatozoon only one sex (the male) 

 is developed. The male frog is hetero-gametic for sex i.e., there 

 are two kinds of spermatozoa, one with and the other without a sex 

 chromosome. If a spermatozoon of the former type enters an egg, 

 a female is produced. If a spermatozoon of the latter type, or no 

 spermatozoon at all, enters an egg, a male is produced. 



It is impossible to enter here into a discussion of the factors which 

 determine sex. While due weight must be given to such morphological 

 distinctions as sex chromosomes in the gametes (or sexual reproductive 

 elements), evidence has been adduced that the fundamental factor may 

 be chemical and metabolic pecularities in the gametes. In pigeons^ .g-., 

 in which the female is the hetero-gametic sex, producing two kinds 

 of eggs it has been shown that the egg which is to develop a female 

 is characterized by a lower metabolism, a lower percentage of water 

 and a higher total content of fat and phosphorus or of phosphatides, 

 than the egg which is to develop a male. There are indications that by 

 changing these chemical conditions sex can be to some extent controlled 

 (Whitman, Riddle). 



The facts of parthenogenesis show that it is not absolutely neces- 

 sary for development that the ovum should have the normal number 

 of chromosomes restored. It can develop with half the number, the 

 chromosomes of the female pronucleus being sufficient for growth, 

 although, of course, in this case for a growth uninfluenced by the 

 properties of the male element. In like manner it is stated that 

 portions of the maturated ovum devoid of a nucleus can undergo 

 development if penetrated by a spermatozoon, the chromosomes of 

 the male pronucleus being sufficient for growth. 



Formation of the Embryo. Not till all these events have taken place 

 extrusion of the two polar bodies, or maturation ; penetration of the 

 spermatozoon, and blending of its head (the male pronucleus) with the 

 remnant of the nucleus of the ovum (female pronucleus), or fecundation 

 not till then does the ovum begin the process of repeated division by 

 which the whole body is reproduced. The fused or segmentation nucleus 

 divides into two, each containing the normal number of chromosomes 

 derived from the splitting of those contributed by both the male and 

 female elements. It is believed that the division takes place in such a 

 way that both male and female chromosomes are represented in each 

 nucleus. The cytoplasm being also cleft by a corresponding furrow. 



