ioo8 A MANUAL OF PHYSIOLOGY 



spermatozoon. When these eggs are afterwards exposed for thirty 

 to forty minutes to 100 c.c. of sea-water, to which 14 or 15 c.c. of a 

 strong solution of sodium chloride (two and a half times the strength 

 of a normal solution, or about i4'6 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. 



The facts of parthenogenesis show that it is not absolutely necessary 

 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. 



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, two complete nucleated cells make their appearance. These 

 divide in turn, till at length (in the mammal) the embryo is repre- 

 sented by a hollow sphere or vesicle, with a cellular crust. During 

 division the upper or outer cells have always been larger than the 

 inner and lower, and have multiplied more rapidly ; and thus it 

 comes about that the hollow sphere of large cells encloses a mass of 

 smaller cells, along with remnants of broken-down yolk and of fluid 

 derived by absorption from the contents of the uterus. The smaller 

 cells continue to multiply and arrange themselves as a lining to the 

 sphere already formed, so that in a short time it becomes double, 

 and we have already differentiated two of the primary embryonic 

 layers the ectoderm, also called the epiblast, or superficial, and the 

 endoderm, also called the hypoblast, or deep layer. The whole sphere 

 is called the blastoderm, or the blastodermic vesicle. 



While this inner shell of endodermic cells is gradually creeping on 

 to completion, there appears at a part where it is already fully 

 formed a small opaque whitish disc, the germinal area or embryonal 

 shield. This represents the stocks on which the framework of the 

 embryo is to be laid down. The area elongates ; at its posterior 

 end appears a thickened line, the primitive streak, soon furrowed by 

 a longitudinal groove, the primitive groove, that marks the direc- 

 tion in which the long axis of the future embryo will lie, but is not 

 itself a permanent line in the building, and ultimately vanishes. 

 The appearance of the primitive streak is the signal that a rapid 

 proliferation of the cells of the germinal area, and especially of the 

 ectoderm, has begun ; and this goes on until a third layer is formed, 

 intermediate in position to the original two, and therefore named 

 the mesoderm. While this is pushing its way over the germinal 



