DEVELOPMENT OF THE EMBRYO. 713 



most, more than 2| or three months ; and there are several cases now on re- 

 cord, in which the interval was from 110 to 170 days, whilst neither of the 

 children was premature in appearance; so that the possibility of a second 

 Conception, when the Uterus already contains an Ovum of several months, 

 can scarcely be denied, however improbable it may seem. 



4. Development of the Embryo. 



934. Under this head it is intended to state, not so much the details of the 

 process of Development, as those leading facts, the knowledge of which is 

 desirable in itself, as well as essential to the due comprehension of the former. 

 It is difficult to see what practical benefit can result from a minute acquaint- 

 ance with all the steps of the evolution of the Embryo, however interesting 

 these may be in a scientific point of view; and the time of the ordinary Stu- 

 dent, on which there are so many pressing calls, may be much better occupied 

 than in committing them to memory. In the following sketch, little will be 

 said respecting the latter stages of the process, or the development of particular 

 organs, since these have been already noticed under their severally distinct 

 heads. Our attention will first be given to the formation of the Embryonic 

 mass, and of the membranes surrounding the Yolk-bag; and then to the ori- 

 gin of the Vertebral column, Digestive organs, and Circulating apparatus. 



935. The Ovum, when it quits the Ovarium, has been stated to contain 

 within the Germinal Vesicle, two cells which did not exist there previously 

 to fecundation; and from each of these, two new cells are subsequently pro- 

 duced, which in their turn give birth to eight others ( 917). In this manner, 

 the number of vesicles originating in the twin-cells of the Germ is continually 

 increased, until at last they become too numerous to be counted, and form a 

 cluster resembling a Mulberry in appearance ; this mulberry-like structure 

 may be conveniently termed the Germinal Mass (Plate I., Fig. 15, ). In the 

 centre of this mass there is found a peculiar Cell, differing from the rest in its 

 greater size, and in possessing a very well-defined annular nucleus, with a 

 pellucid cavity in its centre (Fig. 16, a, b}. From this peculiar Cell, all the 

 parts which enter permanently into the composition of the Embryo are de- 

 veloped ; the vesicles forming the exterior of the germinal mass being sub- 

 servient to a merely temporary purpose. This central or Embryonic Cell is 

 gradually brought to the surface of the Germinal Mass, by the formation of a 

 cavity c in the interior of the latter; for the layer of cells within which this 

 cavity is formed, progressively extends itself, until it comes into contact with 

 the inner surface of the Yolk-bag, having absorbed the yolk into the hollow 

 thus left. Thus out of the periphery of the Mulberry-mass, appears to be 

 formed the exterior layer of what is termed the Germinal Membrane: this 

 membrane is first seen as an epithelium-like layer of cells, covering the Yolk; 

 but beneath this layer, which is afterwards known as the serous lamina of the 

 Germinal membrane, two others are subsequently produced from the central 

 portion of the Germinal mass. Now it is highly interesting to observe, that 

 this Germinal Membrane, which in the higher animals is a mere temporary 

 structure, subservient only to a temporary function, forms, in the lower tribes, 

 the greater part of the permanent fabric of the body. Thus, in the Polypes, 

 the cavity in which the Yolk is inclosed becomes a Stomach ; the external 

 layer of the Germinal Membrane becomes the integument; whilst the internal 

 forms the lining of the Digestive cavity, of which the mouth is formed by 

 absorption of its wall at one point. Here the Yolk is directly absorbed and 

 assimilated by the surrounding membrane. In the higher Oviparous animals, 

 the Germinal Membrane serves to absorb nutritious matter from the Yolk, 

 and to prepare it for the use of the Embryo itself, by converting it into Blood 



60* 



