80 THE BIOLOOT OF AN ANIMAL. 



associated to form tlie segmentation- or cleavage-nucleus ; by this 



act fertilization is completed. 



The process of fertilization appears to be essentially the same among 

 all higher animals, and in a broader sense to be identical with the sexual 

 process among all higher and many lower plants (compare the fern, p. 139), 

 but its precise nature is still in dispute. It is certain that one essential 

 part of it is the union of two nuclei derived from the two respective parents. 

 This has led to the view, now held by many investigators, that inheritance 

 has its seat in the nucleus, and that chromatin (p. 23), is its physical 

 basis. Later researches have shown that another element known as the 

 archoplasm- or attraction-sphere is concerned in fertilization, and this is 

 apparently always derived from the middle-piece. It is not yet certain 

 whether the archoplasm is to be regarded as a nuclear or a cytoplasmic 

 structure, and it is equally doubtful whether it plays an essential or merely 

 a subsidiary role in fertilization and inheritance (cf. p. 84). 



Cleavage of the Fertilized Ovum. Soon after fertilization the 

 ovum begins the remarkable process of segmentation which 

 has already been briefly sketched on j). 25. The segmen- 

 tation-nucleus divides into two parts, and this is followed by 

 a division of the vitellus, each half of the original nucleus becom- 

 ing the nucleus of one of the halves of the vitellus ; that is, the 

 original cell divides into two smaller but similar cells (see Fig. 

 3-1:). These divide in turn into four, and these into eight, and 

 so on, but yet remain closely connected in one mass. In the 

 case of the earthworm, the cells do not multiply in regular 

 geometrical progression, but show many irregularities ; and more- 

 over they become unequal in size at an early period. 



The blastula (pp. 25, 85,) shows scarcely any differentiation 

 of parts, though the cells of one hemisphere are somewhat smaller 

 than the others. From this time forwards the whole course of 

 development is a process of differentiation, both of the cells and of 

 the organs into wliicli they soon arrange themselves. One of 

 the first steps in this process is a flattening of the embryo at the 

 lower pole — i.e., the half consisting of larger cells (Fig. 35, D), 

 The large cells are then folded into the segmentation-cavity so 

 as to form a pouch opening to the exterior ; at the same time 

 the embryo becomes somewhat elongated (Fig. 35, ^, F). 



This process is known as gastrulation^ and at its completion 

 the embryo is called the gastrula. The infolded pouch (called 

 the archenteron) is the future alimentary canal ; its opening (now 

 known as the hlastopoi^e) will become the mouth ; and tlie layei 



