GROWTH AND REPRODUCTION 



773 



frogs. It also sheds light upon the incompleteness and the unlikeness 

 of an embryonal rudiment to a human foetus. The organs existing 

 in ovarian dermoids are rarely of the same completeness as those of 

 parasitic foetuses. This indicates the value of the spermatozoon for 

 the production of a complete individual. Similarly, the eggs of the 

 sea-urchin may be made to develop by placing them in sea-water 

 containing a small amount of magnesium chloride. Nevertheless, 

 from the point of view of heredity, the male pronucleus plays an 

 important part. 



Segmentation. After fertilization, the oosperm becomes fixed in 

 position in the uterus, and then undergoes a series of divisions, first 

 into two cells, then into four, until a mulberry-shaped mass of cells, 

 the morula, or, when large, blastula, is formed (Fig. 479). In this 

 morula a cavity then appears, forming a hollow sphere the 

 blastocyst which is single-layered except in one part. The inner 

 cells of this part then proliferate, and convert the sphere into a 

 double-layered gastrula with a small pore (the blastopore) connecting 

 the cavity with the exterior. The outer of the cell layers is known 

 as the epiblast, the inner as the hypoblast. Between these two 

 layers a third layer develops, known as the mesoblast (Fig. 481). 

 The developing organism now differentiates the various systems 

 concerned in the division of labour of the body. Different systems 

 become evolved from the three layers : 



From the Epiblast. 



From the Mesoblast. 



The epidermis and its de- 

 rivatives e.g., hair, nails, 

 glands, and muscle of 

 sweat glands, etc. 



The epithelium of the nose 

 and mouth, and the 

 glands opening into them ; 

 the anterior lobe of the 

 pituitary gland. 



The central and peripheral 

 nervous systems. 



The supporting tissues of 

 the body: bone, connec- 

 tive tissue. 



The muscles except those of 

 the sweat glands and iris. 



The blood and lymph 

 systems. 



The excretory system ex- 

 cept the epithelia of 

 bladder and urethra. 



The cortex of the supra- 

 renal gland. 



The generative system. 



From the Hypoblast. 



The epithelia of the ali- 

 mentary tract, including 

 the glands entering it. 



The epithelia of the re- 

 spiratory tract. 



The epithelia of the Eu- 

 stachian tube and tym- 

 panum. 



The epithelium of the thy- 

 roid and of the thymus. 



The epithelia of bladder, 

 urethra, and accessory 

 sexual apparatus. 



Implantation. The ovum is usually fertilized in the oviduct, or 

 Fallopian tube. It is then passed by ciliary action into the uterine 

 cavity, where in the morula or blastula stage it embeds itself in the 

 mucous membrane'of the uterus by means of a phagocytic action of 

 its outer layer, which is now known as the trophoblast. The corpus 

 luteum is believed to exert considerable influence through its internal 

 secretion upon the process of implantation. 



Immediately after fertilization of the ovum, the normal mucous 

 membrane of the uterus, the endometrium, undergoes a great increase 



