TYPES OF CLEAVAGE 291 



The first cleavage furrow cuts through the egg along the median axis of 

 the egg, starting at the postero-ventral side of the egg (fig. 1400, E). It, 

 therefore, is a meridional plane of cleavage. The second cleavage plane cuts 

 at right angles to the first plane, producing four equal cells (fig. HOE, F). 

 The third cleavage involves four blastomeres. Its plane of cleavage is almost 

 equatorial but slightly displaced toward the animal pole, and therefore, more 

 truly described as latitudinal plane of cleavage. This cleavage plane divides 

 each of the four blastomeres into a smaller micromere at the animal pole 

 and a larger macromere at the vegetal pole. Eight blastomeres are thereby 

 produced (fig. 140G-I). In certain cases and at least in some varieties of 

 Amphioxus, the four micromeres may not be placed exactly above the macro- 

 meres, but may be rotated variously up to 45 degrees, forming a type of 

 spiral cleavage (fig. 140J). (See Wilson, E. B., 1893.) The fourth cleavage 

 planes are meridional, and all of the eight cells divide synchronously. The 

 result is sixteen cells, eight micromeres and eight macromeres (fig. 140J, K). 

 The fifth planes of cleavage are latitudinal and simultaneous (fig. 140L). The 



Fig. 140. Early cleavage and blastulation in Amphioxus. (K after Hatschek, 1893; all 

 others after Conklin, '32.) (A) Median section through egg in the plane of bilateral 

 symmetry, one hour after fertilization. Second polar body at animal pole; egg and sperm 

 pronuclei in contact in cytoplasm containing little yolk. Approximate antero-posterior 

 axis shown by arrow. D. and V. signify dorsal and ventral aspects of future embryo. 

 MS. = mesodermal crescent. (B) Sperm and egg nuclei in contact surrounded by 

 astral rays. Sperm remnant, SR., shown at right. (C) First cleavage spindle in postero- 

 ventral half of the egg (see fig. 139C). Arrow shows median plane of future embryo 

 and also the median plane of the egg. Observe that the spindle is at right angles to this 

 plane of the egg. (D) Egg in late anaphase of first cleavage. Cleavage furrow deeper 

 at postero-ventral side of the egg. MS. = mesodermal crescent. (E) Two-cell stage. 

 Arrow shows median plane of embryo. MS. = mesodermal crescent now bisected into 

 two parts. (F) Four-cell stage at conclusion of second cleavage, IVi hours after fertili- 

 zation. (G) Four-cell stage at beginning of third cleavage. Posterior cells. P., slightly 

 smaller than anterior cells. (H) Animal pole above, vegetal pole below. Cell at left is 

 posterior, the one at right anterior. Spindles show third or horizontal cleavage plane. 

 (I) Eight-cell stage, IVi hours after fertilization. Posterior cells, below at right, contain 

 most of mesodermal crescent. Arrow denotes antero-posterior axis of embryo. (J) Late 

 anaphase of fourth cleavage. (K) Sixteen-cell stage viewed laterally. There are eight 

 micromeres and eight macromeres. (L) Side view of 32-cell stage, 3'/4 hours after fer- 

 tilization. Every nucleus in anaphase or metaphase of sixth cleavage. (M) Left side of 

 64-ceIl stage. Arrow denotes antero-posterior axis of embryo. (N) Blastula, VA hours 

 after fertilization. Animal pole above, vegetal below. Entoderm cells at vegetative pole 

 are larger, are full of yolk, and are dividing. Blastocoel is large. (O) Eighth cleavage 

 period with more than 128 cells, 4 hours after fertilization. Antero-posterior axis of future 

 embryo shown by arrows. Polar body indicates animal pole of original egg. Dorsal and 

 ventral aspects indicated by D. and V., respectively. MS. — mesodermal crescent. (P) 

 Section of blastula, AVi hours after fertilization. Entoderm cells have nuclei shaded with 

 lines. (Q) Section of blastula, 5'/2 hours after fertilization. (R) Section of blastula, 

 6 hours after fertilization. Mesoderm cells lighter and on each side of entoderm cells. 

 Section nearly transverse to embryonic axis. (S) Section of blastula at stage of pre- 

 ceding but in a plane as in (Q). MS. = mesodermal crescent. (T) Pear-shaped, late 

 blastula. Pointed end is mesodermal; entoderm cells have cross-lined nuclei. D. and V. 

 indicate dorsal and ventral aspects of embryo. See also fig. 167. 



