22 THE INITIATION OF DEVELOPMENT: 



ion with the development of the sperm aster, viz. in the central 

 liquefaction area of the latter. 



According to the classical idea of Boveri, the middle piece 

 of the sperm contained a specific cell organ for division, the 

 centrosome. This acted first as the starting point for the form- 

 ation of the sperm aster in the egg cytoplasm. Next it divided, 

 and an aster developed around each half; together these asters 

 constituted the amphiaster. In other words, development was 

 initiated by the activity of the centrosome carried by the sperm. 



More recently this view has been questioned. It was stressed, 

 especially by Fry, that the centrosome may not exist, at least 

 as a separate plasmatic body. It cannot be denied that the 

 sperm, especially its middle piece, contains a centre which 

 readily gives rise to a gelation (aster) in the egg cytoplasm, 

 but similar asters, so-called cytasters, can be produced at other 

 places in the cytoplasm by a variety of stimuli. They are there- 

 fore certainly not dependent on the presence of a special 

 centrosome. 



Two hypotheses have been advanced, concerning the factor 

 which, in normal fertilisation, is responsible for the occurrence 

 of the amphiaster with its bipolar mitotic spindle. Dalcq 

 suggested the following hypothesis. The sperm, especially its 

 nucleoplasm, is the carrier of the principle that causes "di- 

 centry". The formation, on the disappearance of the nuclear 

 membrane, of a bipolar spindle with its asters, is an inherent 

 property of this nucleoplasm. Dalcq's view is founded on the 

 following observations. (1) Sometimes, in the act of penetrat- 

 ing, the sperm remains stuck with its head in the cortex. In 

 such cases, normal development of the sperm nucleus does not 

 take place. The egg is activated only, and a monastral cycle 

 ensues. This occurs in some cases of heterogeneous fertilisation, 

 e.g. when eggs of the toad Bufo calamita are fertilised with 

 sperm of the newt Triton alpestris (Bataillon). It may also 

 occur when the sperm has been damaged, e.g. by trypaflavin 

 poisoning (Dalcq). (2) If the fusion of the pronuclei is prevent- 

 ed, each may, under certain circumstances, develop separately 

 in such a way that the male pronucleus undergoes a normal 

 mitosis, whereas the female pronucleus gives rise to a monaster. 



