THE EMBRYO. 289 



The ventral plates early split into a deep and superficial 

 part, like the part of the germinal membrane beyond the 

 embryo ; but at their inner part, instead of so splitting, they 

 become thicker and push inwards, so that the plates of oppo- 

 site sides meet together below the chorda dorsalis, and form 

 a mesial plate, in which appear the great blood-vessels 

 and other organs. Among these organs may be mentioned 

 the Wolffian bodies or primordial kidneys (fig. 88), which 

 originally occupy the whole length of the abdominal cavity 

 at the sides of the vertebral column, and are closely con- 

 nected with the development of the reproductive organs; 

 but disappear at a very early period of foetal life, and are 

 replaced by the permanent kidneys, which take origin between 

 and behind them. The space between the superficial and 

 deep divisions of the ventral plates is the great serous cavity 

 of the trunk, subsequently subdivided into the pericardial, 

 pleural, and peritoneal spaces. The superficial division is 

 the source of the cutis and other connective tissues of the 

 visceral wall; while the deep division adheres to the inner 

 layer of the embryo, and completes with it the development 

 of the alimentary tube (Remak). 



The limbs make their appearance from the ventral plates 

 as little buds, which very early display a division into fingers, 

 the thumb or great toe of each lying nearest the head; and 

 subsequently the elongation of the arm and leg takes place. 



211. In the head, the middle layer of the embryo on each 

 side and in front of the chorda dorsalis is called the cephalic 

 plate. We have seen already how it encloses the brain, in 

 the same fashion as the dorsal plates, continuous with it, 

 rise up round the spinal cord. Very soon the brain and 

 parts round it are sharply curved down towards the deep 

 part of the ovum, the margins of the cephalic plate become 

 folded in, and a deep fossa is formed round it, so that the 

 head and neck become separated off from the ovum, while a 

 hood of the outer layer of the germinal membrane rises up 

 over them. In like manner the deep layer of the embryo 

 within the head becomes converted into a cul-de-sac, sur- 

 rounded with the continuation forwards of the mesial plate. 



The margins of the middle layer in the head and neck are 

 thrown into five pairs of processes called bran.chia.1 processes, 

 14 



