FERTILIZATION AND CLEAVAGE 97 



c. Schedule of Development 



A time table of cleavage in a normal batch of Arbacia eggs at 23 °C, is 

 given in Table 2, and photographs on Plates III, IV and V. 



Other schedules of cleavage times in Arbacia punctulata have been 

 given by Fry and Parkes (1934) ; Fry (1936) ; Hoadley and Brill (1937), 

 and Blum and Price (1950 b). The rates of first cleavage at different 

 temperatures and the Q^^q for cleavage as determined by Loeb and 

 Wasteneys (1911a) and Loeb and Chamberlain (191 5) are given in 

 Table 3, together with a few figures by Fry (1936). The rate of the first 

 three cleavages at different temperatures as obtained by Hoadley and 

 Brill (1937) is given in Table 4. 



For a comparison of Arbacia punctulata with some other species, a 

 few references for cleavage rates in other species are given in Table 5. 



TABLE 2 



SCHEDULE OF DEVELOPMENT (e. B. HARVEY) 



Time Table of Cleavage and Development oi Arbacia punctulata Eggs at 23 °C. Times 

 after fertilization for 50 % of the eggs to reach the stage designated. 



Completion of fertilization membrane 2 min. 



Sperm aster 8 min. 



Union of pronuclei (monaster stage) 10 min. 



Completion of hyaline layer (begins 2 min.) 20 min. 



Streak stage 20-35 min. 



Nuclear membrane breaks . 35 min. 



Prophase 35 min. 



Metaphase 40 min. 



Anaphase 42 min. 



Telophase 45 min. 



1st cleavage; 2-cell 50 min.. 



2nd cleavage; 4-cell .... 28 min. after ist 78 min. 



3rd cleavage; 8-cell .... 25 min. after 2nd 103 min. 



4th cleavage; (12-cell) micromeres; 27 min. after 3rd 130 min. 



i6-cell .... 32 min. after 3rd 135 min. 



5th cleavage; (20-cell) .... 22 min. after 4th 157 min. 



(28-cell) .... 30 min. after 4th 165 min. 



32-cell .... 32 min. after 4th 167 min. 



Hatch as blastulae, ca. 1,000 cells 7-8 hrs. 



Gastrulae 12-15 hrs. 



Skeleton begins 19 hrs. 



Plutei I day 



Plutei, maximum without special feeding 3-4 days 



There is a slight variation in cleavage times in different batches of eggs and considerable 

 variation at different times of the year, irrespective of temperature. The above table is for 

 a standard batch in mid-season at 23 °C. It is based on both living eggs and stained sections. 

 There is a greater variation in the later stages of development, e.g., time of hatching. 



