THE DEVELOPMENT OF THE EMBRYO 2i9 



Theoretically, it may be considered that the male transmits form, and 

 the female disjoosition and character, and in man the intellectual power. 

 To this rule there are, however, many very marked exceptions. 



After the disappearance of the germinal vesicle, curious changes in the 

 yolk are perceived, resulting in segmentation. First, depressions or 

 notches are noticed in the membrane surrounding the yolk at two points, 

 and these slowly advance through the mass, cutting it in halves, while 

 almost at the same time a similar process is going on in each half, making 

 four divisions, which are divided again and again, until a mulberry mass is 

 formed. This process of multiplication by division of the mass possesses a 

 remarkable significance, which will be referred to in connection with the 

 process of generation in the lower forms of life (fig. 538). 



Completion of the process of segmentation leaves the yolk a mass of 

 delicate granular spherical masses, each with a clear centre. Conversion of 

 these masses into cells is eifected by the development of an investing mem- 

 brane round each mass. As soon as the cell -formation is perfected, the 

 peripheral cells arrange themselves on the surface of the yolk, the central 

 masses follow, and finally complete the construction of a thick membrane, 

 which is known as the germinal or blastodermic membrane, which soon 

 divides into two layers ; the upper one nearest to the original investure of 

 the yolk, the vitelline membrane, is called the serous layer of the blastodermic 

 membrane, and the lower one the mucous layer. From the upper or serous 

 layer, the outer portions of the animal body, the bones, muscles, and skin, 

 are developed, while the inner or mucous layer, which is in contact with the 

 yolk, forms the internal organs or viscera. 



THE DEVELOP]\IENT OF THE ElilBRYO 



First Signs. — At the outset, the attempt to describe the formation of 

 the various parts of the young animal is met by an insuperable difficulty, 

 because by no form of verbal gymnastics is it possible to describe a whole 

 set of simultaneous processes by the aid of consecutive phrases. It is easy, 

 for example, to state the fact that in the germinal membrane the embryo is 

 formed ; that bones, muscles, integument, and viscera appear, and that 

 adaptive changes go on in the uterus, in which the young one has to pass 

 its embryonic and foetal life; but unless the reader will consent to make a 

 mental effort to realize that the changes are all going on in different degrees 

 at the same time, there is no hope that the writer will succeed in conveying 

 a correct idea of the true nature of the developmental process. 



Proceeding from the point which has just been reached, the formation 

 of a germinal membrane by an accumulation of cells round the inside of the 



