312 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



(3) cleavage of the marginal cells, increasing the number of marginal cells 

 and contributing syncytial nuclei to the yolk floor, 



(4) cleavage within the syncytial mass of the yolk floor, contributing cells 

 to the central cells, such as cell A, figure 153F. 



Eventually a blastular condition is reached as a result of the foregoing 

 cleavages which does not possess an enlarged blastocoelic space; rather the 

 blastocoel is in the form of scattered spaces within a loosely aggregated cap 

 of cells (fig. 153G). This blastula might be regarded as a stereoblastula, i.e., 

 solid blastula (Dean, 1896; Whitman and Eycleshymer, 1897). 



6) Lepisosteiis (Lepidosteiis) osseiis. The early development of the 

 gar pike, Lepisosteus osseus, another bony ganoid fish, resembles that of Amia 

 described above. The disc of protoplasm which takes part in the early cleav- 

 ages is a prominent mass located at one pole of the egg (fig. 154A). The first 

 two cleavage furrows appear to be meridional and partly cleave the proto- 

 plasmic cap of the egg, as indicated in figure 154B-D. The next cleavages 

 are vertical and somewhat parallel to one of the meridional furrows (fig. 

 152E). The fourth cleavages are vertical, cutting off four central cells from 

 the peripherally located marginal cells (fig. 152F). As in Amia, the marginal 

 cells contribute syncytial nuclei to the yolk bed below the protoplasmic cap, 

 and these in turn contribute definite cells to the growing blastodisc. The 

 blastula of Lepisosteus consists of a loosely aggregated cap of cells among 

 which are to be found indefinite blastocoelic spaces (fig. 152G). (See Dean, 

 1895.) 



7) Gymnophionan Amphibia. Cleavage presumably is holoblastic, result- 

 ing in a disc of small micromeres at the animal pole, with large, irregular 

 macromeres, heavily yolk laden, located toward the vegetal pole (fig. 182A). 

 (See Svensson, '38.) The latter cells become surrounded during gastrulation 

 by the smaller micromeres (Brauer, 1897). The blastula of the gymnophionan 

 amphibia essentially is solid and may be regarded as a stereoblastula. 



3. Meroblastic Cleavage 



The word meroblastic is an adjective which refers to a part of the germ; 

 that is, a part of the egg. In meroblastic cleavage only a small portion of the 

 egg becomes segmented and thus gives origin to the blastoderm. Most of the 

 yolk material remains in an uncleaved state and is encompassed eventually 

 by the growing tissues of the embryo. A large number of vertebrate eggs 

 utilize the meroblastic type of cleavage. Some examples of meroblastic cleav- 

 age are listed below. 



a. Egg of the Common Fowl 



(Note: As cleavage in reptiles resembles that of birds, a description of rep- 

 tilian cleavage will not be given. The reader is referred to figure 231, con- 



