TYPES OF CLEAVAGE 319 



tral mass of cells. (2) Vertical cleavages arise whose furrows extend 

 peripherad toward the margin of the disc. These furrows and previously 

 formed, similar furrows now approach the outer edge of the blasto- 

 disc (germinal disc). (See figs. 155H; 156D). (3) True latitudinal 

 or horizontal cleavages occur which serve to provide lower cell bound- 

 aries for the more peripherally located, central cells (see cell A, fig. 

 156E), and which also contribute nuclei without cell boundaries to 

 the disc substance in this immediate area (see cell B, fig. 156E). As 

 a result, the marginal or peripheral areas of the blastodisc around the 

 mass of completely formed, central cells are composed of: (a) mar- 

 ginal cells which appear near the surface of the blastodisc, having 

 partial boundaries at the blastodisc surface, and {b) a deeper-lying 

 protoplasm, possessing nuclei without cell boundaries. This deeper- 

 lying, multinucleated, marginal protoplasm constitutes a syncytium 

 (fig. 156F). 



2) Formation of the Periblast Tissue. As indicated above, the activities 

 of the blastoderm extend its margins peripherad. In so doing, some of the 

 mitotic divisions in the peripheral areas contribute nuclei which come to lie 

 in the deeper portions of the blastodisc. Some of these nuclei wander distally 

 and yolkward into the more peripherally located, uncleaved portions of the 

 protoplasm below the enlarging primary segmentation cavity or blastocoel. 

 A syncytial protoplasm containing isolated nuclei thus arises around the pe- 

 ripheral margin of the blastoderm in its deeper areas. This entire syncytial 

 protoplasm, composed of a continuous cytoplasm with many nuclei, is known 

 as periblast tissue. It is made up of two general areas: ( 1 ) the peripheral 

 periblast around the margin of the blastodisc and (2) a central periblast 

 below the primitive blastocoel (fig. 156G). This periblast tissue is a liaison 

 tissue which brings the yolk and the growing mass of cells of the blastodisc 

 into nutritive contact. 



When this condition is reached, two kinds of embryonic tissues exist: 



(a) the formative or embryonic tissue proper, composed of an aggrega- 

 tion of distinct cells. These cells constitute the cellular portion of the 

 blastoderm (see blastodisc cells, fig. 156G), and 



(b) the peripheral and central periblast tissue (see fig. 156G). The latter 

 functions as a trophoblast tissue, and it is continuous with the seg- 

 mented portion of the blastoderm around the peripheral areas of the 

 blastodisc. Centrally, however, it is separated from the segmented 

 area of the blastoderm by the primary blastocoelic cavity. The devel- 

 opmental condition at this time may be regarded as having reached 

 the primary blastular stage. 



3) Morphological Characteristics of the Primary Blastula. This condition 

 of development is reached while the egg continues in the uterus (fig. 157G). 



