. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 332 CLEAVAGE (SEGMENTATION) AND BLASTULATION POSIirON OF 2ND. MATURATION SPINDLE POINT OF SPERM PENETRATION. D. CLEAVAGE FURROW OF FIRST CLEAVAGE ON PART OF NUCLEUS WHICH ENTERED " BRIDGE' Fig. 165. Drawings of cleavage of a partially constricted egg of Triturus viridescens, illustrating delayed nucleation. (Slightly modified from Fankhauser, '48.) (A) Shows constricting loop, point of sperm entrance, and second maturation spindle. The constr
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 332 CLEAVAGE (SEGMENTATION) AND BLASTULATION POSIirON OF 2ND. MATURATION SPINDLE POINT OF SPERM PENETRATION. D. CLEAVAGE FURROW OF FIRST CLEAVAGE ON PART OF NUCLEUS WHICH ENTERED " BRIDGE' Fig. 165. Drawings of cleavage of a partially constricted egg of Triturus viridescens, illustrating delayed nucleation. (Slightly modified from Fankhauser, '48.) (A) Shows constricting loop, point of sperm entrance, and second maturation spindle. The constricted portion to the right will contain the fusion nucleus. (B) First cleavage furrow in right half of egg. (C) Second cleavage. The nucleus in the "bridge" area has migrated into the "; (D) Third cleavage. The nucleus in the bridge area has divided and pro- duced cleavage furrow through the bridge cytoplasm as indicated. One of the daughter nuclei of this cleavage is now in the constricted part of the egg at the left. (E) Fourth cleavage = first division of left half. (F) Blastular stage—late blastula at right, middle blastula at left. ectoderm grows over the half of the embryo which failed to develop. Also, the notochord rounds up into a normally shaped notochord but is only half the normal size. Essentially, however, these separated blastomeres develop into "half embryos in which some cells have grown over from the uninjured to the injured side, but in which absolutely no change has taken place in the potency of the individual cells or of the different ooplasmic substances" (Conklin, '06). Similarly, at the four-cell stage isolation of anterior and poste- rior blastomeres gives origin to anterior and posterior half embryos respectively. The developing sea-urchin egg has been used extensively for experimental work in the study of isolated blastomeres. In figures 163 and 166A-D are shown the different developmental possibilities which ari
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