TYPES OF CLEAVAGE 



315 



MARGINAL CELLS 



SYNCYTIAL PROTOPLASM 



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NUCLEUS OF PANDE R 

 BLASTODI SC CE LLS 



K/^W. 



ZONE OF JUNCTURE 



(SYNCYTIAL GERM WALL) 



CENTR A L PERIBLAST 



PRIMI T I VE BLASTOCOEL 



CE LLULA R GER M WAL L 



MARGI NAL PERI BLAST 



MARGIN OF OVERGROWTH 



Fig. 156. Cleavage in chick blastoderm, sectional views. (After Patterson, '10.) (A) 

 Median section through blastoderm approximately at right angles to furrow shown in 

 fig. 155 A. (B) Section through blastoderm of about eight-cell stage. (C) Section 

 through blastoderm, showing 32 cells, also showing horizontal cytoplasmic cleft (seg- 

 mentation cavity). (D) Median section through blastoderm similar to that shown in 

 fig. 155E. (E) Median section through blastoderm similar to that of fig. 155G. (F, G) 

 Diagrammatic views of developing avian blastoderms. (F) Diagrammatic section and 

 surface view of chick blastoderm shown in fig. 155G and fig. 156E. (G) Section of 

 chick blastoderm about time that egg is laid, depicting the primary blastocoel below 

 the blastoderm and syncytial tissue at the margins. Observe that the syncytial tissue 

 serves to implant the blastoderm upon the yolk substance. 



furrow consists of a slight meridional incision near the center of the blastodisc, 

 cutting across the disc to an extent of about one half of the diameter of the 

 latter (fig. 155A). This furrow passes yolkward but does not reach the lower 

 portion of the disc where the cytoplasm is filled with coarse yolk granules 

 (fig. 156A). The second cleavage occurs about 20 minutes later and consists 

 of two furrows, one on either side of the first furrow and approximately at 

 right angles to the first furrow. These furrows may be regarded as meridional 

 (fig. 155B). Though both of the second furrows tend to meet the first furrow 

 at its midpoint, one of the second furrows may be displaced and, hence, may 

 not contact the corresponding furrow of the other side. The third set of 

 furrows is vertical, cutting across the second set of meridional furrows, and, 

 consequently, tends to parallel the first cleavage furrow (fig. 155C). The 

 fourth set of furrows is also vertical and, although not synchronous, it pro- 

 ceeds gradually to form eight central cells which are surrounded by twelve 



