94 



John H. Gerould, 



Second Cleavage and Four -cell Stage. 



The spindles of the second cleavage (Fig, 20) are markedly 

 laeotropic, and are situated nearer the active than the vegetative 

 pole. They are present simultaneously in the two blastomeres only 

 for a very short time, the smaller cell, AB, dividing, in Ph. vulgare, 

 from one to five minutes earlier than the larger, CD. In Ph. gouldii 

 the difference is sometimes even greater. 



Fig:. A. 



4-cell stage in Phascolosoma gouldii, viewed from the side. 320 : 1. 



The cleavage furrows (Fig. A and Fig. 21) are obliquely 

 meridional, and three of the resulting cells, A, B, and C, are of 

 approximately the same size, w'hereas D has perhaps five times 

 the volume of each of the others. C, however, is slightly larger in 

 Ph. gouJdii than A or B (Fig. A). 



At the end of this stage, the cell A swings up obliquely to the 

 left over D, until it touches C at the surface along a line which 

 0. Hertwig (1880) and others have called a polar furrow (Fig. B), 

 while its displacement to the left allows the lower extremity of B 

 to come into contact with the relatively huge cell, D, along another 

 polar furrow. The two furrows are at right angles to each other. 

 The lower furrow, however, is not at the vegetative pole and 

 directly opposite the upper, as is true of the eggs of many animals, 

 but is somewhat on the side of the egg. It is not a prominent 

 line in Phascolosoma, and I have found it to be of no value as a line 

 of orientation. There is less mobility in the torsion of the cells in 



