ORGAN-FORMING SUBSTANCES IN EGGS OF ASCIDIANS. 22/ 



DESCRIPTION OF PHOTOGRAPHS. (PLATE XI.) 



All the photographs are of the living eggs and embryos of Cynthia (Styela~) partita 

 in sea water and are magnified 112 diameters. Prints from the original negatives 

 were arranged in order and photographed and the plate is an actual print (Roto- 

 graph Process) from this negative. These photographs are not diagrams but like 

 the specimens themselves they require and will repay careful study. That portion of 

 the egg which appears darkest in the photographs is the orange-yellow mesoplasm, that 

 which is lightest is the transparent ectoplasm, while that which is apparently inter- 

 mediate in shade between these two is the slate-gray endoplasm. The contrast 

 between these substances is therefore greater in reality than appears in the photo- 

 graphs. Every egg or embryo is inclosed in a transparent chorion which does not 

 show in the photographs ; within the chorion and around the periphery of the egg are 

 numerous " test cells" which contain yellow pigment. 



PHOTO i. Egg about fifteen minutes after the entrance of the sperm showing the 

 yellow protoplasm as a dark cap at the lower pole where the sperm lies. The clear 

 protoplasm is the light zone above the yellow ; the rest of the egg is gray. 



PHOTO 2. Egg about twenty minutes after the entrance of the sperm; the egg 

 substances are shown as in the preceding photograph. 



PHOTO 3. Egg about thirty minutes after the entrance of the sperm, seen from the 

 right side. The yellow protoplasm is moving to the posterior pole and forming the 

 crescent there ; the clear protoplasm lies chiefly above (ventral to) the crescent ; in 

 the middle of the crescent and at the periphery of the egg is a small spot of clear 

 protoplasm (caudal chymoplasm) which first appears around the entering sperm and 

 ultimately goes into a pair of caudal mesenchyme cells. 



PHOTO 4. Egg about thirty-five minutes after the entrance of the sperm, showing 

 the yellow crescent as a dark band with a clear area through its center ; the latter is 

 the first cleavage spindle. Both the crescent and the periphery of the egg show a 

 slight notch in the lower (ventral) border, which is the beginning of the first cleav- 

 age furrow. The egg is somewhat obscured by overlying test-cells which here and 

 elsewhere give it a mottled appearance ; high focus. 



PHOTO 5. Egg about forty minutes after the entrance of the sperm, viewed from 

 the posterior ventral pole. The cleavage furrow is deepest in the region of the cres- 

 cent. Above the crescent is the clear (ventral) ectoplasm. High focus. 



PHOTO 6. Egg about forty-five minutes after the entrance of the sperm. Two-cell 

 stage viewed from the animal pole showing the yellow crescent at the posterior 

 margin of the egg. 



PHOTO 7. Stage similar to the preceding, viewed from the posterior pole ; below 

 the crescent is shown the gray endoplasm of the vetegal pole, above the crescent the 

 clear ectoplasm of 'the animal hemisphere, in the furrow at the middle of the cres- 

 cent a small amount of clear chymoplasm. 



PHOTO 8. Stage similar to the preceding, viewed from the right side (an end view 

 of one of two cells), showing the clear ectoplasm in the upper (animal) hemisphere, 

 the yellow crescent v mesoplasm) at the posterior pole, the light gray crescent (chorda- 

 neuroplasm) at the anterior pole and the dark gray endoplasm between the two cres- 

 cents at the lower pole. The definitive localization of these substance is complete at 

 this stage. 



