GROWTH, REGENERATION AND REPRODUCTION 1119 



equal amounts of chromatin of dual origin, now approach one another 

 and are joined or even fuse into one which is known as the cleavage 

 or segmentation-nucleus. Shortly afterward the nuclear membrane 

 disappears, a spindle is developed and a number of chromosomes 

 arise from the cleavage-nucleus which in all probability have been 

 derived in equal proportions from the two germ-nuclei. Fertilization 

 is then rapidly followed by the division of the cell, the stimulus for 

 it having been given by the centrosome which thus becomes the 

 controlling agent in the further development of the embryo. 



The successive divisions now following eventually give rise to numerous cells 

 which arrange themselves in the form of either a spherical mass (morula) or a 

 circular disc. In '.he former case, the center finally becomes hollow, forming the 

 blastula. These cells then arrange themselves as a uniform layer which is known 

 as the blastoderm. Somewhat later the blastula is invaginated (gastrula), thereby 

 giving rise to two layers of cells, namely, an outer or ectoderm, and an inner or 

 entoderm. The next step in this development is the formation of a third or 

 median layer which is known as the mesoderm. In this way, the foundation is 

 laid for a physiological division of labor, because from these three layers are de- 

 rived the various organs of the adult individual. But the question of whether 

 this mesoderm arises from the entoderm or from the ectoderm, has not been defi- 

 nitely settled as yet; in fact, it seems that it may originate from either. This 

 differentiation of the germinal layers having been completed, genesis begins. The 

 ectoderm or epiblast eventually gives rise to the central nervous system and the 

 epidermal tissues, while the mesoderm or mesoblast originates the vascular, mus- 

 cular and bony tissues as well as the generative and excretory organs, exclusive 

 of the bladder, the first part of the male urethra and the female urethra. The 

 entoderm or hypoblast forms the epithelium of the intestines as well as that of the 

 intestinal glands and respiratory passage, the prostatic portion of the male urethra 

 and the entire female urethra. 1 



Parthenogenesis and Artificial Parthenogenesis. Sexual repro- 

 duction necessitates the conjugation of two cells and the fusion of 

 their nuclei. During this process the number of the chromosomes in 

 the germ-cells is reduced to one-half the number characteristic of the 

 somatic cells. 2 In a few instances, however, the ovum alone is capable 

 of producing a new individual ; but this mode of reproduction, which is 

 known as parthenogenesis, remains confined to the simplest forms, 

 such as the insects and the lower crustaceans and rotifers. It should 

 also be noted that in some species parthenogenesis alternates with 

 sexual generation, but the variability of the non-sexual offsprings is as 

 great as that of the sexual ones. This fact speaks against the concep- 

 tion of Weissman, according to which the purpose of sexual reproduc- 

 tion is to induce variations. 



In parthenogenesis the stimulus is given by the second polar body 

 which thus takes the place of the spermatozoon. The ovum develops 



1 For a more detailed discussion of the process of fertilization the reader is 

 referred to textbooks on Embryology, and especially to such books as Wilson's 

 "The Cell in Development and Inheritance." This brief account has been 

 inserted here merely to serve as a connecting link between the substance of this 

 chapter and that of the succeeding. 



2 Van Beneden, Arch, de Biologic, iv, 1883. 



