Mosaic Development in Ascidian Eggs. 211 



remainder of the egg.^ For the sake of brevity this earliest form 

 of locaHzation may be described as concentric or spherical, 

 although the germinal vesicle does not lie exactly in the center of 

 the egg but is slightly eccentric toward one pole. 



During maturation and fertilization this concentric localization 

 gives place to a polar or radial form. Immediately after the 

 entrance of the spermatozoon into the egg the peripheral layer of 

 yellow mesoplasm flows rapidly to the lower pole where it collects 

 in the form of a cap; the clear ectoplasm which escapes from the 

 germinal vesicle at first lies at the animal pole where it surrounds 

 the maturation spindles but after the entrance of the spermatozoon 

 it also flows to the lower pole where it collects into a layer or 

 stratum just above the mesoplasm; the gray endoplasm after these 

 movements occupies almost all of the upper half of the egg. The 

 egg at this stage appears to be radially symmetrical, the three 

 principal substances being arranged in strata at right angles to the 

 egg axis. 



Soon after the entrance of the spermatozoon this radial form of 

 localization gives place to a bilateral one; the sperm nucleus and 

 aster move up to the equator of the egg along one meridian which 

 further development shows to be the median plane on the posterior 

 side; the clear and yellow substances also move to the posterior 

 pole along with the sperm nucleus and the yellow substance here 

 forms a crescent around the posterior side of the egg, just below 

 the equator. At this stage the egg is bilaterally symmetrical, 

 there being but one plane which will divide equally all of the 

 ooplasmic substances. 



Finally during the' first cleavage this early bilateral localization 

 is changed into the definitive localization which is characteristic of 

 all stages up to the late gastrula. The yellow crescent remains in 

 the position which it occupied before the first cleavage and here 

 gives rise to muscle and mesenchyme cells; the clear protoplasm 

 comes to occupy most of the ventral hemisphere and gives rise to 

 ectoderm; the gray substance occupies the dorsal hemisphere in 



'Although I have not been able to isolate these various ooplasmic substances before cleavage begins 

 and, therefore, can bring no experimental evidence to prove that they are organ-forming substances at 

 this early stage, it nevertheless seems probable that materials which are identical in color and texture 

 with the organ-forming substances of later stages, to which they directly give rise, are also similar in 

 potency. There is no apparent reason for believing that these strikingly different kinds of ooplasm 

 of the ovarian egg are anv less distinct or more nearly totipotent than during the cleavage stages. 



