108 EXTERNAL FACTORS III. 5 



perature is not so harmful as a longer exposure to a less degree 

 of cold. The processes of maturation and fertilization were 

 both abnormal. Granules of chromatin took the place of the 

 tetrads and were unequally distributed to the spindle-poles ; or, 

 if the chromosomes (tetrads) had been normally formed before 

 the commencement of the experiment, their division was 

 irregular, in extreme cases all passing to one pole and into the 

 first polar body. Again, the formation of the polar bodies 

 might be suppressed altogether, or abnormal, the second only 

 being formed, or both as one, or the first polar body might be 

 as large as the egg itself. The achromatic figure was also 

 deformed, the spindle being split at one or both poles (pseudo- 

 triaster, pseudo-tetraster), and centrosomes appeared instead of 

 the usual centrosomal granules. The cytoplasm became granular, 

 the vitelline membrane was not formed, two or more eggs 

 frequently fused together. Polyspermy, with consequent multi- 

 plication of asters and centrosomes, was very noticeable, and, in 

 fertilization stages, a separate pronucleus may be formed from 

 each female chromosome, or fragment of a chromosome. 



Closely connected with the cytoplasmic effects brought about 

 by these temperature changes is the phenomenon of artificial 

 parthenogenesis, produced by Morgan and Greeley in Arbacia 

 and Asterias by lowering the temperature of the sea-water to 

 the freezing-point. Greeley has also shown that a lowering of 

 the temperature, like the raising of the osmotic pressure, results 

 in a withdrawal of water, the cause to which, as is well known, 

 Loeb attributed the development of unfertilized ova in his 

 experiments. 



Greeley has shown that by the combination of a low tem- 

 perature with a chemical reagent a higher percentage of 

 swimming blastulae can be obtained. 



LITERATURE 



C. B. DAVENPORT and W. E. CASTLE. Studies in morphogenesis ; 

 III. On the acclimatization of organisms to high temperatures, Arch. 

 Ent. Mech. ii, 1896. 



H. DKIESCH. Entwicklungsmech. Stud. IV : Experimentelle Veran- 

 derung des Typus der Furchung und ihre Folgen (Wirkungen von 

 Warmezufuhr und Druck), Zeitschr. iciss. ZooL Iv, 1893. 



