REPRODUCTION 867 



these changes there has been much discussion. It seems to be agreed 

 that the spindle is formed in part, at any rate, from the nucleus or 

 germinal vesicle, and that the result of the process is the expulsion 

 of a portion of the chromatin skein (p. 18), which is restored by the 

 male pronucleus when it arrives and penetrates the ovum.* 



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), ox fecundation not till then 

 does the ovum begin to divide. The germinal spot, or nucleus, splits 

 into two, and the yolk 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 represented 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 con- 

 tinue 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 

 epiblast) or superficial, and the hypoblast, or deep layer. The whole 

 sphere is called the blastoderm, or the blastodermic vesicle. 



While this inner shell of hypoblastic 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 direction 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 epiblast, has begun ; 

 and this goes on until a third layer is formed intermediate in position 

 to the original two, and therefore named the mesoblast. While this 

 is pushing its way over the germinal area and into the rest of the 

 blastodermic vesicle, the epiblast in front of the primitive streak rises 

 up in two lateral ridges, enclosing between them the medullary 

 groove. The medullary groove is the beginning of the cerebro-spinal 



* Loeb has lately made a contribution of far-reaching importance to 

 our knowledge of the significance of fertilization by the discovery that the 

 unfertilized egg of the sea-urchin can develop in sea-water to which 

 magnesium chloride has been added in a certain proportion, just as if a 

 spermatozoon had entered the egg. From his experiments he deduces 

 the conclusion that the unfertilized egg of the sea-urchin contains all the 

 elements essential for development, and that ' the only reason which 

 prevents it from developing parthenogenetically is the constitution of the 

 sea- water.' 



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