BLASTODERMIC MEMBRANE. 589 



in texture ; but we shall see hereafter that all the future organs 

 of the body, however varied and complicated in structure, arise out 

 of it, by modification and development of its different parts. 



The segmentation of the vitellus, moreover, and the formation 

 of the blastoderm ic membrane, take place in essentially the same 

 manner in all classes of animals. It is always in this way that 

 the egg commences its development, whether it be destined to 

 form afterward a fish or a reptile, a bird, a quadruped, or a man. 

 The peculiarities belonging to different species show themselves 

 afterward, by variations in the manner and extent of the develop- 

 ment of different parts. In the higher animals and in the human 

 subject the development of the egg becomes an exceedingly com- 

 plicated process, owing to the formation of various accessory 

 organs, which are made requisite by the peculiar conditions under 

 which the development of the embryo takes place. It is, in fact, 

 impossible to describe or understand properly the complex embry- 

 ology of the quadrupeds, and more particularly that of the human 

 subject, without first tracing the development of those species in 

 which the process is more simple. We shall commence our descrip- 

 tion, therefore, with the development of the egg of the frog, which 

 is for many reasons particularly appropriate for our purpose. 



The egg of the frog, when discharged from the body of the female 

 and fecundated by the spermatic fluid of the male, is deposited in 

 the water, enveloped in a soft elastic cushion of albuminous sub- 

 stance. It is therefore in a situation where it is freely exposed to 

 the light, the air, and the moderate warmth of the sun's rays, and 

 where it can absorb directly an abundance of moisture and appro- 

 priate nutritious material. We find accordingly that under these 

 circumstances the development of the egg is distinguished by a 

 character of great simplicity; since the whole of the vitellus is 

 directly converted into the body of the embryo. There are no acces- 

 sory organs required, and consequently no complication of the 

 formative process. 



The two layers of the blastodermic membrane, above described, 

 represent together the commencement of all the organs of the foetus. 

 They are intended, however, for the production of two different 

 systems ; and the entire process of their development may be ex- 

 pressed as follows : TJie external layer of the blastodermic membrane 

 produces the spinal column and all the organs of animal life; while the 

 internal layer produces the intestinal canal, and all the organs of vege- 

 tative life. 



