EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT 49 



the process continues there comes into existence a ball of cells, more or 

 less hollow in the center, called a blastula; g in Fig. 4.2 represents a 

 blastula sliced open to reveal its internal cavity, the cleavage cavity. 

 Essentially, a blastula is an embryo composed of a single layer of cells, as 

 shown in the figure. This one-layered stage then proceeds to convert itself 

 into a two-layered embryo, the gastrula (/). Gastrula formation occurs in 

 a variety of ways, various expedients being resorted to if the presence of 

 yolk impedes the process. Fig. 4.2 shows the relatively simple process 

 possible when the cleavage cavity is not obstructed with yolk. One side of 

 the blastula swings inward (/?); this in-bending, accompanied by con- 

 tinued cell division, suffices to produce the condition shown at /. 



With attainment of the gastrula stage an embryo shows forecasts of 

 things to come. The cells remaining on the outside constitute a layer called 

 the ectoderm, obviously in position to form the outer surface of the body. 

 The cells which fold inward form a layer called the endoderm lining 

 the newly formed cavity, the archenteron. The latter is the beginning of the 

 digestive tract. The archenteron has but one opening to the exterior, the 

 blastopore. In many invertebrates the blastopore becomes the mouth, at 

 or near the anterior end of the body. In vertebrates, on the other hand, 

 the region of the blastopore becomes the posterior or tail end of the body, 

 though usually the blastopore itself does not remain as the posterior open- 

 ing of the digestive tract, the anus. 



Almost at once a third layer, the mesoderm, forms; although this is not 

 shown in Fig. 4.2, it can be visualized as located in the remnant of the 

 cleavage cavity, between the ectoderm on the outside and the endoderm 

 on the inside. 



The ectoderm gives rise to the external surface of the body, including 

 such things as skin, scales, feathers, hair, and to the nervous system and 

 the sensory membranes of the sense organs. The endoderm hnes the diges- 

 tive tract and gives rise to glands associated with digestion, such as liver 

 and pancreas. The lungs of land-dwelling vertebrates also arise from the 

 endoderm. The mesoderm forms almost everything else: muscles, bones, 

 kidneys, connective tissue, and so on. 



The pattern of development illustrated above may be said to consist of 

 the following sequence: (1) single cell; (2) successive cell divisions to 

 form clusters of two, four, eight, sixteen, and so on, cells; (3) a one- 

 layered stage; (4) a two-layered stage; (5) a three-layered stage. The 

 uniformity of occurrence of this pattern of development throughout the 

 animal kingdom, from worms to man, is remarkable. 



At least two factors must be operative in production of this uniformity. 



