PHALLUSIA MAMILLATA 395 



Photo 1 Equatorial section of an egg at the beginning of the second cleavage 

 the nuclei and surrounding protoplasmic areas are in process of division, as is 

 shown by the dark (blue) areas in the middle of each blastomere; the mesodermal 

 crescent is shown at the posterior (lower) edge of each blastomere as a dark (red) 

 cap; the anterior (upper) margin of the egg is rendered indistinct by the presence 

 of test cells. 



Photo 2 Section of an egg in the 2-cell stage, parallel with the first cleavage 

 plane; with the stain used the central protoplasmic area is but faintly stained, 

 but the mesodermal crescent, which is cut transversly, is shown as a deeply stain- 

 ing cap at the lower left margin of the photograph; the latter is oriented so that 

 the posterior pole is to the left, the anterior to the right, the ventral pole above, 

 and the dorsal pole below. 



some cases, perhaps in all, the region of the egg from which the 

 chorda and neural plate arise is stained more deeply with the 

 haemalum than are the surrounding areas (fig. 1.) 



The substances are arranged in the unsegmented egg and are 

 later distributed to the various blastomeres in exactly the same 

 manner as in Cynthia and Ciona. The first cleavage plane 

 coincides with the plane of bilateral symmetry; the second 

 divides two posterior blastomeres, containing the mesodermal 

 crescent, from two anterior ones, containing the materials which 

 go into the future neural plate and chorda; the third separates 

 the vegetative halves of these four cells from the animal halves, 

 in such manner that the mesodermal crescent and the endoder- 

 mal areas are left in the vegetative half of the egg. 



In all the later cleavages, the form and relative size of the 

 blastomeres and the distribution of the various ooplasmic sub- 

 stances to these blastomeres are apparently precisely the same 



