Exercise XXIII 



FERTILIZATION AND EARLY DEVELOPMENT 121 



greatly with the temperature. An approximate 

 schedule of development at 23°C for an East 

 Coast sea urchin, Arbacia punctulata, is as 

 follows: 



Other species of sea urchin have different 

 schedules of development, usually slower. 



After penetrating the jelly coat, a sper- 

 matozoon touches the surface of the egg. At 

 this point an entrance or fertilization cone forms, 

 within about 20 seconds, which engulfs the 

 sperm head. The cone is very difficult to see, 

 so don't be disappointed if you miss it. A 

 fertilization membrane also begins immediately 

 to form around the egg, and to lift off, leaving 

 a space between it and the egg surface. This 

 takes about 2 minutes. The changes that take 

 place in the egg during the next 40 minutes or 

 so are difficult to see in v/vo, but slides are 

 available showing sections of eggs in various 

 stages of mitosis. 



Depending upon the species and the tempera- 

 ture, about 45 to 90 minutes after fertilization, 

 sea urchin eggs begin to undergo their first 

 cleavage. Prepare a sample containing 30 to 50 

 eggs, and record the time at which the first 



eggs have cleaved, and then at intervals of 2-3 

 minutes, record how many eggs have cleaved 

 until all that are going to have done so. Draw 

 a graph showing the percentage of cells that 

 have undergone first cleavage (ordinate) against 

 time in minutes (abscissa). Draw another graph 

 showing the percentage of cells that have cleaved 

 per one- or two-minute interval, i.e., the rate 

 of cleavage (ordinate) against time in minutes 

 (abscissa). 



The latter curve usually has the typical bell- 

 shaped form of a "population curve," the dis- 

 tribution of any measured property in a popula- 

 tion of independent individuals. The former 

 curve (usually S-shaped or sigmoid) is the 

 typical summed-over or integral form of a 

 population curve. If in this class, for example, 

 you measured everyone's height, and then 

 plotted two curves — one of the number of per- 

 sons in each height range (66-68 inches, 68-70 

 inches, and so on) as ordinate against the height 

 as abscissa, the other curve recording the total 

 number of persons under each height as ordinate, 

 against the height as abscissa — you would prob- 

 ably obtain a similar pair of curves. (To say 

 this in the language of calculus: the bell-shaped 

 distribution curve is the differential form; the 

 sigmoid curve is its integral.) 



In order to determine the schedule of develop- 

 ment through the first four cleavages, you will 

 want to examine fertilized eggs during the first 

 five or six hours after fertilization. Your 

 instructor will provide you with two batches of 

 eggs, fertilized 3 hours and 1.25 hours before 

 the laboratory period begins. With these two 

 batches and the eggs you have fertilized at the 

 beginning of the period, you will have samples 

 of fertilized eggs at all stages during the first 

 six hours of development. Working in pairs, set 

 up a sampling schedule so that you can follow 

 the progress of development at half-hour inter- 

 vals, using the time of fertilization of the three 

 batches as starting times. Thus, using the egg 

 fertilized 3 hours before the laboratory session 

 began, you may get time intervals up to 6 hours 

 after fertilization. A sample should be taken 

 by placing a few drops of eggs selected randomly 



