172 



GEOLOGICAL BIOLOGY. 



within the organism of separate individuals, is the next essen- 

 tial step in the process, and the ovule, thus fertilized (as the 

 result of this conjunction is called) proceeds under proper 

 conditions to further develop, and when sufficiently developed 

 for independent life is thrust out of the parental organism, 

 is separated, and becomes a separate individual, as repre- 

 sented by the stages, d-g, Fig. 47. The distinction of sex is 

 again represented in the new-born individual which is born 

 already differentiated (2 g, Fig, 47) in this respect, and as it 

 matures develops the organization of either a male or a female 

 individual, and only as thus differentiated is the continuation, 

 of the process of reproduction possible. 



With the third stage of cell-development, above described, 

 begin the processes of cellular differentiation, or histogenesis, 

 within the walls of the cellular organism. The segmentation 

 of the contents of the interior of the ovule is the first step in 

 this process, and results in the formation of innumerable 



spherules. The cell in this con- 

 dition is called a blastiila, or 

 morula (according to the extent 

 and mode of its segmentation). 

 The blastiila results when the 

 segmentation affects only a part 

 of the cell-contents, and a hol- 

 low ball-like cell is formed ; in 

 the vionila the whole cell- 

 contents are segmented, or, at 

 least, the unaffected part is 

 relatively very small, and tlie 

 result is a solid ball of cellules 

 (Fig. 48). 



Fourth. The next stage of development is the formation 

 into a gastrula, in which specialization of the secondary 

 spherules or cellules take place, and an outer and an inner 

 layer are formed. The typical gastrula is formed by the 

 dimpling in of the hollow sphere of the blastosphere to form 

 a two-layered cell (Fig. 49). 



The Primitive Tissues, Endoderm, Ectoderm, and Mesoderm. — 

 The Ectoderm and the Endoderm are the primitive undifferen- 



FlG. 49. 



-Gastrula stage of the ovum. 

 McMurrich.) 



(After 



