TYPES OF CLEAVAGE 305 



forms directly by cell arrangement and not by the development of a cleft 

 within the trophoblast cells, as in the eutherian mammals. (See fig. 147C-E; 

 compare with figs. 144G; 145J.) 



The descriptions of the mammalian blastocysts presented above pertain 

 only to the primary condition of the blastocyst. The changes involved in later 

 development, resulting in the formation of the secondary blastocyst, will be 

 described in the next chapter which deals specifically with blastulation. 



(For more detailed descriptions of early cleavage in the metatherian and 

 eutherian mammals see: Hartman ('16) on the American opossum; Hill ('18) 

 on the opossum from Brazil; Hill ('10) on the Australian native cat, Dasyurus 

 viverrinus; Heuser and Streeter ('29), and Patten ('48) on the pig; Lewis and 

 Gregory ('29), Gregory ('30), and Pincus ('39) on the rabbit; Huber ('15) 

 on the rat; Lewis and Wright ('35 ) and Snell ('41 ) on the mouse; Lewis and 

 Hartman ('41) and Heuser and Streeter ('41) on the Rhesus monkey.) 



b. Holoblastic Cleavage of the Transitional or Intermediate Type 



Contrary to the conditions where small amounts of yolk or deutoplasm 

 are present in the egg of the higher mammal or in Amphioxus, the eggs of 

 the vertebrate species described below are heavily laden with yolk. As the 

 quantity of yolk present increases, the cleavage phenomena become less and 

 less typically holoblastic and begin to assume meroblastic characteristics. 

 Hence the designation transitional or intermediate cleavage. 



1) Ambystoma maciilatum ( punctatiim). The newly spawned egg of 

 Arnbystoma maculatum is nearly spherical and measures about 2 mm. in 

 diameter, although the egg size is somewhat variable. The animal pole con- 

 tains within its median area a small depression, the "light spot" or "fovea." 

 Within the fovea is a small pit harboring the first polar body. (A comparable 

 pit is shown in the frog's egg, fig. 119C.) After the second polar body is 

 formed, this pit may appear somewhat elongated, and the light spot disap- 

 pears. Just before the first cleavage, the animal pole appears flattened similar 

 to the condition in the frog's egg. The flattened area soon changes to an 

 elongated furrow which progresses gradually downward toward the opposite 

 pole (fig. 149A, B). This cleavage furrow is meridional, dividing the egg into 

 two, nearly equal blastomeres. The second cleavage furrow is similar to the 

 first but at right angles to the first furrow (fig. 149C). However, considerable 

 variation may exist, and the second furrow may arise at various angles to the 

 first, dividing each of the first two blastomeres into two, slightly unequal, 

 daughter blastomeres. The third set of cleavages is latitudinal, and each 

 blastomere is divided into a smaller animal pole micromere, and a larger 

 vegetal pole macromere (fig. 149D). Later cleavages may not be synchronous. 



The first three cleavages described above conform generally to the rules 

 of typical holoblastic cleavage. However, from this time on cleavage digresses 

 from the holoblastic pattern and begins to assume certain characteristics of 



