TYPES OF CLEAVAGE 297 



blastocoel or segmentation cavity becomes an enlarged space filled with fluid, 

 displaced toward the animal pole (fig. 143B-D). The contained fluid of the 

 blastocoel of amphibia is alkaline, according to the work of Buytendijk and 

 Woerdeman ('27), having a pH of 8.4 to 8.6. 



For general references regarding cleavage in the frog, see Morgan (1897); 

 Pollister and Moore ('37); Rugh ('51); and Shumway ("40). 



c. Cyclostomata 



Cleavage in the eggs of the genera of the family, Petromyzonidae, resembles 

 very closely that of the frog. Further description will not be included. How- 

 ever, in the marine cyclostomes or hagfishes, the cleavage phenomena are 

 strongly meroblastic. (See description of the marine cyclostomatous fish at 

 the end of this chapter. ) 



2. Atypical Types of Holoblastic Cleavage 



A variety of cleavage types is found in the eggs of many vertebrate species 

 which do not follow the symmetrical, ideally holoblastic pattern exhibited in 

 the egg of Amphioxus or even in the egg of the frog. In all of these atypical 

 forms the entire egg ultimately is divided by the cleavage furrows with the 

 possible exception of the eggs of the bony ganoid fishes, Amia calva and 

 Lepisosteus osseiis (and also in certain of the gymnophionan amphibia). In 

 the latter species the yolk material at the yolk-laden pole of the egg is invaded 

 by isolated nuclei which form a syncytium in the yolk material. Eventually 

 this yolk material is formed into definite cells and incorporated into the gut 

 area of the embryo. 



a. Holoblastic Cleavage in the Egg of the Metatherian and Eutherian 



Mammals 



1) General Considerations. The eggs of metatherian and eutherian mam- 

 mals are the most truly isolecithal of any in the phylum Chordata. They have 

 also a cleavage pattern distinct from other chordate eggs. The first cleavage 

 plane in the higher mammalian egg very often divides the egg into a larger 

 and a slightly smaller blastomere (figs. 144B; 145A, F; 1478, J). As shown 

 by the work of Heuser and Streeter ('41 ) in the pig, the smaller blastomere 

 is destined to give origin to the formative tissue of the embryo's body, while 

 the larger blastomere gives rise to auxiliary tissue, otherwise known as the 

 nourishment-obtaining or trophoblast tissue (fig. 145A-E). The smaller blas- 

 tomere also contributes some cells to the trophoblast tissue. A similar con- 

 dition of progressive specialization of the smaller and the larger blastomeres 

 of the two-cell stage, producing two classes of cells, the one mainly formative 

 and the other auxiliary or trophoblast, is present in the monkey (Heuser and 

 Streeter, '41 ) and probably in other higher mammals as well. 



If one compares the early history of these two blastomeres with the early 



