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TEXTBOOK OF ZOOLOGY 



the animal to the vegetal pole, so that instead of being spherical it is 

 a mere flattened slit. The circumference of the placula corresponds 

 to the equator of the coeloblastula much as if two saucers were placed 

 with their cavities together. The amphihlastula of a sponge (Fig. 56) 

 is a free-swimming larval stage, the cells of the animal half being 

 sharply distinguished from those of the vegetal half by their smaller 

 size and ciliated covering. The superficial blastula, as already de- 

 scribed, is formed when the dividing plasma islands reach the sur- 

 face of the egg and form a blastoderm covering it. The discoblastula 

 results from the repeated divisions of the disc on the surface of these 

 eggs, and is in the form of a disc several cells in thickness. 



I 



Fig. 417. — Unequal stereoblastula of the moilusk Crepidula (boat shell). (Re- 

 printed by permls.sion from Outline of Comparative Embryology by Richards, pub- 

 lished by John Wiley and Son.s, Inc., after Conklin.) 



Examples of these various types of blastulae occur as follows : equal 

 eoeloblastulae are found among the echinoderms and in amphioxus; 

 unequal eoeloblastulae (Fig. 288), in the frog, annelids (Fig. 110) and 

 molluscs; stereoblastulae, in those forms having spiral cleavage in 

 which there is a large amount of yolk; morulae, chiefly in the 

 coelenterates ; placulae, in ascidians and in some nematodes; amphi- 

 blastulae, limited to the sponges; superficial blastulae, chiefly among 

 the arthropods, and discoidal blastulae, of the same disposition as 

 discoidal cleavage. 



The formation of the blastula marl^s the end of the first great period 

 of embryological development. It is followed by germ-layer forma- 

 tion which takes place in two steps; the first, endoderm formation 

 or gastridation (Fig. 416), and the second, the formation of primary 

 germ layers (primary ectoderm and primary endoderm), from which 

 are derived the definitive germ layers, ectoderm, mesoderm, and endo- 



