XXIII 



FERTILIZATION AND EARLY 

 DEVELOPMENT; CONTINUATION OF 

 THE GENETICS EXPERIMENT 



(Readings: Weisz, pp. 532-536; 594-601. 

 420-421; 430-432.) 



S.P.T., pp. 335-340. Villee, pp. 



During fertilization, a haploid sperm nucleus 

 fuses with a haploid egg nucleus to form the 

 diploid nucleus of a new cell, which by repeated 

 mitoses and differentiation develops into an 

 adult organism. All the somatic cells of the 

 adult organism, including the precursors of the 

 mature germ cells (spermatocytes and oocytes) 

 have the double chromosome number (2/2). As 

 part of the process of maturation of the germ 

 cells, this is halved in the reduction division of 

 meiosis. The sperms that engage in fertilization 

 are wholly mature and haploid. In most ani- 

 mals, however, the egg does not complete its 

 maturation until after the sperm head has 

 entered it. The egg nucleus then completes its 

 meiosis, throwing off the supernumerary nuclei 

 in one or two polar bodies and achieving the 

 haploid condition just before fusing with the 

 sperm nucleus. In many coelenterates and 

 echinoderms, the egg has finished its matura- 

 tion divisions before the sperm enters, so that 

 fusion of nuclei and cleavage can proceed 

 immediately. 



THE SEA URCHIN 



During this laboratory period we shall ob- 

 serve fertilization and the first stages of develop- 

 ment in an echinoderm, the sea urchin. The 

 sea urchin can be induced to shed its eggs and 

 sperm by passing a weak electric current through 

 it or by injecting a small quantity of potassium 

 chloride solution; or the ovaries and testes can 

 be removed by dissection. It has been estimated 

 that one sea urchin contains about 10'^ sperm, 

 or about 8 million eggs. 



Procedure 



Obtain a petri dish containing a suspension of 

 eggs in sea water, and examine them under the 

 dissecting microscope at convenient magnifica- 

 tions. Note the thick jelly coat that holds the 

 eggs apart. Add one drop of dilute sperm sus- 

 pension to the eggs, swirl gently to mix, and 

 record the time and room temperature. 



The schedule of sea urchin development varies 

 slightly in different batches of eggs and varies 



120 



