DEVELOPMENT OF EMBRYONIC MEMBRANES OR LAYERS 389 



From the mesohlast proceed — 



(h) The generative and urinary organs (the epithelium of the bladder and urethra excepted). 

 (i) The voluntary and involuntary muscles (the muscular fibres of the sweat glands excepted). 

 (/) The vascular and lymphatic systems, serous membranes, and spleen. 

 (k) The skeleton and connective tissues. 



From the hypoblast proceed — 



(I) The epithelium of the alimentary canal and the glands which open into it. 



(to) The epithelium of the urinary bladder. 



(w) The epitheUum of the Eustachian tube, the tympanum, the bronchial tubes, and the air sacs of the lungs. 



(o) The epithelium Uning the vesicles of the thyroid body, and the epitheUal nests of the thymus. 



Home investigators, following His, have still further divided the blastoderm. They describe the true mesoderm 

 as consisting of two epitheUal lamellae. According to them the complete blastoderm would consist of four layers ; 

 namely, epiblast, outer or somatic mesoblast, inner or splanchnic mesoblast, and hypoblast. 



The separation of the blastoderm into layers is followed by other changes in rapid succession. Very soon blood- 

 vessels and a rudimentary heart, composed of nucleated cells (minus muscular fibres and nerves) make their appear- 

 ance (Plate xc. Figs. 1, 3, 4, and 5). The heart, as already stated, is formed prior to its containing blood. The organ 

 is made before it is called upon to discharge its peculiar function. The same holds true of other organs — the lungs, 

 for example. The formation of organs before they are called upon to discharge specific functions affords the strongest 

 possible evidence of design. This evidence is strengthened when it is borne in mind that the organs and systems are, 

 in many cases, simultaneously produced. It is not a mere case of successive developments, but of successive and 

 synchronous, continuous developments ; the Master- Architect and Builder carrying out His work in part piecemeal 

 and in part by simultaneous, complicated, combined operations. 



The heart is not only made before it contains blood, but it also assumes peristaltic rhythmic movements for 

 propelling the blood when it reaches its cavities. The blood, therefore, is not the cause of the movements of the heart, 

 as is generally beUeved. As a matter of fact, the heart, before it contains either muscular fibres, nerves, or blood, 

 opens and closes with time-regulated beat, and quite independently. The same holds true of the so-called contractile 

 vesicles in plants and animals. This is due to the circumstance that the heart is a containing as well as a propelling 

 and transmitting organ. All the hollow viscera (stomach, intestine, bladder, uterus, &c.), even in the adult, are 

 endowed with similar powers. They are all containing as well as propelling organs. 



The changes which occur in the heart and vascular system during the process of development and in the transi- 

 tion from the foetal to the adult state are of the most complicated and startling description, and will be adverted to 

 further on when speaking of the placental, foetal, and adult circulations in man. 



The developing chick lives upon the albuminous and other materials in its vicinity, plus a little moisture and 

 air. Organ after organ appears in anticipation of the function to be discharged by it. The brain and nerves, 

 alimentary canal, glands, lungs, lymphatics, muscles, bones, feathers, &c., are all developed in a comparatively 

 short period, and in advance of the peculiar work to be performed by each. The incubating process over, the 

 completed bird emerges from what is practically an empty shell into the outer world, and immediately sets about 

 procuring extraneous food. 



If the development of the chick within the egg is marvellous, and a triumph of design infinitely greater than 

 that achieved by the watchmaker when he laboriously constructs a watch, the advent of the fully-fledged chick on 

 terra firma and its instant and eager pursuit of food are, in a sense, still more wonderful. What an extraordinary 

 combination of foresight and power does the development and birth of a chick and its immediate search for food as 

 an independent being imply. All the functions of its multiple existence are at once brought into play : its lungs 

 breathe air, its alimentary canal digests food, its muscular and nervous systems enable it to run here, there, and 

 everywhere, its blood circulates, its glands secrete and excrete, and its brain and sense organs connect it in the most 

 effective way with the outer world, in which it is henceforth to hve and move. Shall we be told, or if told, shall we 

 believe, that the fully-developed living chick is the result of accident— a mere matter of spontaneous generation— a 

 product of irritability and artificial stimulation, or forsooth of environment ? 



