316 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



marginal cells. In figure 155D, five central cells are shown, while in figure 

 157E, eight central cells are present. The central cells do not have boundaries 

 below and, thus, are open toward the yolk. As a result, their protoplasm is 

 continuous with the protoplasm in the deeper-lying portions of the disc. The 

 marginal cells have boundaries only on two sides, and the cleavage furrows 

 which form the sides of the marginal cells continue slowly to extend in a 

 peripheral direction toward the margins of the disc (fig. 155D). The egg 

 is in this stage of development when it leaves the isthmus and enters the uterus 

 (fig. 157A, F). 



Cleavage from this point on becomes very irregular, but three sets of fur- 

 rows are evident: 



(a) There are vertical furrows which extend peripherad toward the margin 

 of the blastodisc. These furrows meet at various angles the previously 

 established furrows which radiate toward the periphery of the blasto- 

 disc (in fig. 155E, see a., b., c). A branching effect of the radiating 

 furrows, previously established, in this manner may be produced (in 

 fig. 155E, see c). 



(b) Another set of vertical furrows is found which cut across the median 

 (inner) ends of the radiating furrows. The latter produce peripheral 

 boundaries for the centrally located cells (see fig. 155E, d., e., f. ). The 

 central cells thus increase in number as the blastodisc extends periph- 

 erally. As a result of this set of cleavage furrows, a condition of the 

 blastodisc is established in which there is a mass of central cells, 

 having peripheral boundaries, and an area of marginal cells which 

 lies more distally between the radiating furrows. It is to be observed 

 that the marginal cells lack peripheral boundaries (fig. 155E, F). 



(c) A third and new kind of cleavage, cytoplasmic but not mitotic, now 

 occurs below the centrally placed cells, namely, a latitudinal or hori- 

 zontal cleft which establishes a lower boundary for the centrally lo- 

 cated cells with the subsequent appearance of a blastocoelic space filled 

 with fluid (fig. 156B, C). 



Thus, at the 16- to 32-cell stages (fig. 155D, E) some of the more cen- 

 trally located central cells have complete cellular boundaries (fig. 156C), 

 but central cells, located more peripherally, may not have the lower boundary. 

 The marginal cells also lack a lower boundary. 



A little later, at the 60- to 100-cell stages (fig. 155F), the chick blastoderm 

 presents the following characteristics: 



(a) There is a mass of centrally located cells. These cells lie immediately 

 above the horizontal cleft mentioned above (fig. 156C, D). They are 

 completely bounded by a surface membrane and represent distinct 

 cells. These cells continue to increase by mitotic division and, as early 

 as the 64-cell stage (fig. 155F), the centrally located cells are in the 



