of Human Monstrosity . 237 



germinative membrane essentially different: and to each is appro- 

 priated the office of laying- the groundwork of distinct portions of 

 the embryo, and to a certain point, of perfecting these. But no 

 part of the embryo is finally completed without at least one of 

 the other parts afterwards participating in the work. 



The first perceptible indication of the embryo is a darker line 

 in the middle of the transparent area. It is situated on the infe- 

 rior surface of the serous membrane, and indicates the future 

 spine of the animal. On this same membrane, on either side of 

 the primitive streak, the dorsal plates are disposed, which meet in 

 such a way as gradually to form a cavity for the future spinal 

 cord and brain ; then, on either side of these, are the ventral 

 plates. As the process advances, the ventral plates meet in front 

 only, and the head is bent downwards towards the yolk. So that 

 at this stage of the process, the rudiments of the whole motive 

 and sentient systems, are formed on the serous portion of the blas- 

 toderma; whilst its mucous portion, lying below the serous, is the 

 open abdominal space, and rests immediately on the yolk. 



In the mean time the granules of the intervening vascular 

 layer are more developed. They are collected together more 

 closely in the dark border, and in smaller streaks near the pel- 

 lucid area. As they come in mutual contact, a change takes 

 place in their component parts. Their external portions coalesce 

 to form canals which include the more fluid parts: thus, gradu- 

 ally, are formed on the vascular membrane a number of small 

 veins which anastamose and separate to form a fine net work, 

 and then finally open into the large external circular vein formed 

 similarly in the dark border, and called the 'terminal vein.' The 

 circle is not completed at that part which is above the anterior 

 extremity of the embryo, but either limb of the arc terminates 



