TYPES OF CLEAVAGE 321 



nomena. Cleavage in the majority of these fishes simulates that of the bird, 

 but certain exceptional features are present. In some, as in Torpedo ocellata, 

 meroblastic cleavage is present in an extreme form. The zygotic nucleus di- 

 vides and the two daughter nuclei divide again forming a syncytial state before 

 the appearance of the first cleavage furrow. The tendency of retardation or 

 suppression of the cytoplasmic mechanism of cleavage which occurs in the 

 bird blastoderm thus is carried to an extreme form in the early development 

 of some elasmobranch fishes. 



The first cleavage furrow is meridional or nearly so (fig. 158A), and the 

 second furrow is similar and at right angles to the first furrow. The third set 

 of furrows is vertical and meets the previous furrows at various angles. The 

 fourth set of cleavages is vertical and synchronous, as is the preceding, and 

 gives origin to three or four central cells, which, on surface viewing, have 

 complete cell boundaries but below their cytoplasms are confluent with the 

 cytoplasm of the blastodisc (fig. 158B and E). Around the periphery of these 

 central cells, are on the average ten marginal cells which have their cytoplasms 

 confluent below and peripherally with the general cytoplasm of the disc. The 

 fifth cleavage furrows are mixed. That is, in the central part of the disc the 

 cleavage furrows are latitudinal, as the mitotic spindles in this area form 

 perpendicular to the surface. As a result, distinct daughter cells are cut off 

 above, while the daughter cells below have cytoplasms confluent with the 

 general cytoplasm of the disc. A blastocoelic cavity appears between these 

 two sets of central cells. In the marginal areas the fifth set of cleavages is 

 vertical, cutting off more central cells and giving origin to more marginal cells. 

 The sixth set of cleavages is a mixture of vertical cleavages at the periphery 

 and latitudinal cleavages centrally; it produces a condition shown in figure 

 158F. In surface view, the blastoderm appears as in figure 158C, D. 



From this time on cleavage becomes very irregular and a developmental 

 condition soon is produced which possesses a central blastoderm of many 

 cells with an enlarged blastocoelic cavity below (fig. 158G). A syncytial peri- 

 blast tissue is present at the margins of the blastoderm which also extends 

 centrally below the blastocoelic space where it forms a central periblast (fig. 

 158G). In this manner, two kinds of cells are produced: 



(a) a blastoderm of distinct cells which ultimately produces the embryo 

 and 



(b) a surrounding trophoblast or periblast tissue which borders the yolk 

 substance peripherally and centrally. As in the chick, the periblast 

 tissue has nutritive (i.e., trophoblast) functions. 



2) Problem of the Periblast Tissue in Elasmobranch Fishes. Two views 

 have been maintained, regarding the origin of the periblast nuclei in the 

 elasmobranch fishes. One view maintains that they arise from the accessory 

 sperm nuclei derived from polyspermy, for polyspermy is the rule here as it 



