REPRODUCTION AND DEVELOPMENT IN CHORDATES 



5.13). The process of engulfing the spermatozoon requires only a few seconds 

 in many animals, although it may take as long as an hour. Mo.st often the 

 tail of the spermatozoon is not taken into the egg; only the head containing 

 the male pronucleus and the middle piece containing the centriole, which 

 gives rise to the centrioles of the spindle of the zygote, are involved in fertiliza- 

 tion (Fig. 5.8). If the tail of the spermatozoon enters the egg, it undergoes 

 degeneration as development begins; this happens in the frog, the bat, and 

 some other animals. 



Development 



Reproduction in the vertebrates is complete when maturation, activation, 

 and amphimixis have taken place, when the differentiated egg and sperm 

 cells have united. The zygote is potentially a new individual; it has the 

 capacity to develop into an organism similar to its parents in all essential 

 respects. The process of development, which always follows reproduction by 

 syngamy in the multicellular animals, consists essentially of cell division, 

 cell localization, and cell differentiation. Growth occurs in all developing 

 individuals and is responsible for the increase in the amount of protoplasm, 

 which is correlated with the increase in cell numbers by continued cell divi- 

 sion. Development is a continuous process in which a definite series of events 

 occurs in a definite sequence under very limited environmental conditions. 

 Food and oxygen are furnished, waste products are removed, excessive loss of 

 water is prevented, and a rather limited temperature range is maintained. 

 The methods of caring for the metabolic requirements of a developing in- 



Obcyte nucleus 



Entrance x^tT^ 



cone 



Fig. 5.13. Entrance of sperma- 

 tozoon and formation of first 

 meiotic spindle in the egg of 

 Thalassema (cf. Fig. 5.12); x820. 



Sperm 



145 



