388 DESIGN IN NATURE 



§ 77. Development of the Embryonic Membranes or Layers from which the future Being proceeds. 



The division and subdivision of the ovum inaugurates what is practically a process of weaving ; the result 

 being the production of a continuous membrane, known as the blastodermic layer, which in turn admits of division. 

 This layer first divides into two and constitutes the external and internal blastodermic layers. At this stage the 

 egg presents the appearance of a globular sac enveloped in three concentric membranes, namely, the structureless 

 vitelUne membrane, the external blastodermic membrane composed of cells, and the internal blastodermic membrane, 

 also composed of cells. The cavity of the egg is occupied by a transparent fluid. 



The process of segmentation of the vitellus and the formation of the blastodermic membranes are the most 

 important processes in the development of the egg. By these the vitellus or yolk, composed originally of an albu- 

 minous matter, plus oily granules and protoplasm, is converted by a series of marvellous transformations into the 

 several organs and body of the future chick. From the blastodermic membranes all differentiation proceeds. The 

 segmentation of the vitellus and the formation of the blastodermic membranes is essentially the same in all animals, 

 whether the ultimate product be a fish, a reptile, a bird, or a quadruped ; the distinguishing features of the 

 several classes of animals being due to the development of accessory structures in particular directions. These, 

 in the human subject, as will be shown presently, are numerous and important. 



The two layers of the blastodermic membrane represent the commencement of all the organs of the foetus ; 



the external blastodermic membrane produc- 

 ing the spinal column and the organs of 

 animal life ; the internal blastodermic mem- 

 brane producing the intestinal canal and the 

 organs of vegetative life. 



The external blastodermic membrane, as 

 organisation proceeds, presents an elongated 

 oval thickening (the embryonic spot), with 

 wide opaque edges enclosing a lighter portion 

 (area pellucida) with a delicate central furrow 

 (the primitive trace). The opaque edges grow 

 upwards to form the dorsal plates which en- 

 velop the spinal canal, and outwards and 

 downwards to form the ventral plates and 

 ribs which envelop the abdominal cavity. In 

 this way what are virtually two tubes or 

 the one containing the brain and spinal cord ; the other the intestinal track or canal 



Fig, 

 brane 



f9. — A. Impregnated egg showing first traces of embryo. .', Vitelline mem- 

 c, vitellus or yollc ; d, primitive trace in area pellucida ; .% embryonic spot. 



B. Transverse section of egg at an early stage of development, a, External layer 

 of blastodermic membrane ; b, b', dorsal plates ; c, internal layer of blastodermic 

 membrane. 



C. A similar section more advanced, a, External layer of blastodermic membrane ; 

 b, b', dorsal plates united and enclosing sjiinal canal ; c, c', abdominal plates ; rl, section 

 of spinal column, with lamina and ribs ; e, internal layer of blastodermic membrane 

 (after Dalton). 



cylinders are produced, 

 (Fig. 79, A, B, C). 



Of late years the blastoderm has been divided into three layers, namely, an outer, which is the epithelial mem- 

 brane of the blastodermic vesicle (Kauber's layer) ; an inner, the primitive entoderm ; and an ill-defined middle stratum 

 consisting of somewhat granular cells. The three layers were supposed by B. v. Beneden to form the three permanent 

 layers of the blastoderm ; but this view has been disputed by Rauber, Kolliker, Lieberkuhn, and Heape, who 

 maintain that in the rabbit and mole the middle stratum at this stage of development blends with Rauber's layer 

 to form the frimitive ectoderm. According to Bonnet there is in the sheep an addition to the middle layer from the 

 peripheral thickened portion of the hypoblast, the cells of which are supposed to form the connective tissue and 

 blood. " The appearance of a middle layer causes the originally bi-laminar blastoderm to be tri-laminar, and 

 its three layers have received the names of ectoderm, mesoderm, and entoderm ; or epiblast, mesoblast, and 

 hypoblast." 



The following table gives briefly the structures derived from the three layers in question : — 



From the epiblast proceed — 



(a) The nervous system, consisting of the brain, spinal cord, peripheral nerves and sympathetic nerves. 



(b) The epithelial structures of the organs of special sense. 

 (0) The epidermis and its appendages (hair and nails). 



(d) The mammary, sebaceous, and sweat glands, with the muscular fibres of the latter. 



(e) The epitheUum of the mouth and of the glands opening into it ; the enamel of the teeth, &c. 



(/) The epitheUum of the nasal passages, part of the pharynx, and the cavities and glands opening into the 



nasal passages. 

 {g) The epithelium of the anus, vagina, and urethra. 



