Fig. 19.5. Early stages in the development of Amphioxus. The egg is almost 

 microscopic but has practically no yolk. The processes of development are direct 

 and easier to follow than those of the frog whose eggs contain so much yolk, a, 

 fertilized egg. Egg and sperm nuclei in contact. The minute second polar body is 

 at the top; the first one has disintegrated, b, two-celled stage with nuclei dividing 

 again, c, four-celled stage, two hours after fertilization. Note a temporary cavity 

 (segmentation c.) formed as the cells divide, d, eight-celled stage, a side view, 

 showing the smaller cells at the upper or animal pole, the future front end of the 

 animal, e, all the cells are dividing at nearly the same rate which would not occur 

 if any of them contained much yolk. The nuclei are in metaphase and anaphase 

 stages of division. The segmentation cavity, open at one end, is traced by a broken 

 line. /, g, h; blastula stages. Cells multiply and the embryo grows. Its cavity, the 

 blastocoel, is shown in the half section. /, gastrula. The embryo flattens on the side 

 that is finally its posterior end. This is called the gastrulation or stomach forming 

 stage. /, k, I; views into the right half of the embryo. /, the embryo is now shaped 

 like a broad raspberry; the two layers of its wall are of ectoderm that will form the 

 skin and nervous system, and endoderm that becomes the lining of the alimentary 

 canal, now an open cavity, called the archenteron. The blastocoel is squeezed out 

 of existence, k and /, the embryo is growing longer; at the rear, its walls draw to- 

 gether except for the small anal opening. Layers of cells, the mesoderm, have 

 spread out between the ectoderm and endoderm. Mesoderm will become skeleton, 

 muscle, blood and other tissues. (After Conklin. Courtesy, Hegner and Stiles: 

 College Zoology, ed. 6. New York, The Macmillan Co., 1951.) 



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