EMBRYOLOGY OF CRYPTOBRANCHUS 467 



imately parallel to the second cleavage furrow, extending verti- 

 cally and increasing the number of macromeres instead of cutting 

 off micromeres (figs. 75 to 80). Thus while the total number 

 of cells is always sixteen, the number of micromeres varies from 

 four to eight. 



We thus get as one extreme type an approximately latitudinal 

 fourth cleavage furrow; as the other extreme a fourth cleavage 

 furrow divided into two separate grooves, one on each side of 

 the second furrow and approximately parallel to it and to each 

 other. Between these two extremes we find examples of all 

 possible intermediate conditions. 



With regard to the manner of fourth cleavage, eggs of this 

 stage may be classified into five types, depending on the num.ber 

 of micromeres present. For the purpose of such a classification, 

 irregularities in the third cleavage must be allowed for: in cases 

 where the third cleavage has come in diagonally or latitudinally 

 to cut off a small cell, such a cell is divided by the fourth cleavage 

 into two small cells, of which onlv the one nearer the anim.al 

 pole is to be counted as a micromere (fig. 77) . 



To determine the mode, twenty -five eggs were examined in 

 the sixteen-cell stage, and the results tabulated as follows: 



Number of micromeres 8 7 6 5 4 



Number of cases 4 7 8 3 3 



The table shows that the most frequent manner of cleavage 

 is intermediate between the two extremes described. 



In the majority of cases the micromeres are arranged with 

 considerable regularity ifl two parallel rows, separated by the 

 second cleavage furrow (see especially figs. 78 and 80). This 

 is the necessary result of the biradial symmetry instituted by 

 the normal mode of third cleavage, providing there is no exten- 

 sive shifting of the micromeres. The condition reminds one of 

 the cleavage pattern of the corresponding stage of the teleost 



egg. 



Through a shifting of the micromeres, the biradial symmetry 

 of the cleavage pattern of the blastodisc is usually interfered 

 with (figs. 75 to 81). In this region, the first and second cleav- 



