A text-book of dental histology and embryology, including laboratory directions . ion of the egg of Ascaris megalocephala, var. bivalens. (Boveri.) A, thespermatozoon has entered the egg; its nucleus is shown at tf; beside it lies thegranular mass of archoplasm (attraction sphere); above are the closing phases inthe formation of the second polar body (two chromosomes in each nucleus). B, germnuclei (9, cf) in the reticular stage; the attraction sphere (a) contains the dividingcentrosome. C, chromosomes forming in the germ nuclei; the centrosome divided. D, each germ nucleus resolved iDto chrom
A text-book of dental histology and embryology, including laboratory directions . ion of the egg of Ascaris megalocephala, var. bivalens. (Boveri.) A, thespermatozoon has entered the egg; its nucleus is shown at tf; beside it lies thegranular mass of archoplasm (attraction sphere); above are the closing phases inthe formation of the second polar body (two chromosomes in each nucleus). B, germnuclei (9, cf) in the reticular stage; the attraction sphere (a) contains the dividingcentrosome. C, chromosomes forming in the germ nuclei; the centrosome divided. D, each germ nucleus resolved iDto chromosomes; attraction sphere (a) double. E, mitotic figure forming for the first cleavage; the chromosomes (c) already split. F, first cleavage in progress, showing divergence of the daughter chromosomestoward the spindle poles (only three chromosomes shown) (Wilson.) 346 EARLY STAGES OF EMBRYOLOGY The lines of cell division proceed in a regular way, theplanes passing in such direction as to multiply the numberof cells by two in each set of divisions. Very soon the cells Fig. 263. Four stages in the development of amphioxus, illustrating the formation of thegastrula. I. The blastula, a hollow sphere of cells; those at the lower pole largerthan those at the upper and filled with yolk granules. II. Invagination of thelower pole, because of more rapid growth of cells at the upper pole. III. Thegastrula, complete invagination; the creature is now a two-layered bag. A spaceshould be shown between the layers: bl, the mouth of the bag, or blastopore;hy, inner layer of cells—hypoblast; ep, outer layer of cells—epiblast. IV. Thegastrula will now eongate; the cavity becomes the alimentary canal; the blastoporethe orifice at one end. HYPOBLAST 347 around the black pole show a tendency to divide morerapidly than those at the white pole. At this stage theindividual is made up of a hollow sphere of cells with aspace at the centre, the cells at the upper surface beingsmall and rapidly dividi
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectteeth, bookyear1912
