Smithsonian Report, 1896. 



Plate X! 



F. O. 



< ' bl. fo 





a. c. s. 



4. C. 5. 



Embryo 



Heart 



Processes of Life Revealed by the Microscope. 



Fig. 2. Various phases in the reproduction of one of the higher animals. In the upper 

 series is shown the fusion of the father and mother germ cells; in the middle series appear 

 some of the earlier phases of segmentation of the fertilized ovum. The lower figure (modi- 

 fied from Marshall) represents a medisection of an amphibian embryo sufficiently far advanced 

 to show that the original cells into which the ovum divided have differentiated and arranged 

 themselves in such a manner as to form the beginnings or protons of the great systems of 

 organs — brain, enteron, and heart. 



O, Ovum; n, nucleus of the ovum. § This sign indicates that the ovum is a mother or 

 female germ cell. Z, Zoosperm. cf Sign indicating that the zoosperm is a father or male 

 germ cell. / pn, Female pronucleus; m pn, male pronucleus. These two pronuclei fuse and 

 form the nucleus of the true reproductive cell, the fertilized ovum. In the two figures at the 

 right both signs ( 9 d") are used to indicate that both germ cells are represented in each figure. 



F O, Fertilized ovum. That is the true reproductive cell, composed of a father or male and 

 a mother or female germ cell fused. The steps of the fusion are shown in the upper series. 

 s n, Segmentation nucleus. 2 c s, Two-cell stage; that is, the fertilized ovum has divided 

 once, forming two, (Compare the reproduction of the amoeba.) 4 c s, Four-cell ctage. bl, 

 Blastula stage, in which the fertilized ovum has divided into very many cells, all remaining 

 together. 



Embryo.— The division of the organs has proceeded very far, and the cells have begun to 

 differentiate and form organs: Brain; ch, body axis, or notochord; Enteron, or alimentary 

 canal, and Heart. 



