104 EXTERNAL FACTORS III. 5 



that at the low temperatures in question growth may occur without 

 differentiation (Fig. 55). Thus in one series of experiments at 

 24-5 for six days the blastoderm increased in diameter from 

 4-4 mm. (the average diameter of the blastoderm in unincubated 

 eggs) to 6-9 mm. The primitive streak was, however, not formed. 



A temporary exposure to low temperatures often inflicts 

 a permanent injury on the egg and leads to malformations. 

 Kaestner, by subjecting the eggs at many different stages to 

 temperatures of 15-25, 10 and 5, has discovered that the 

 capacity of resistance decreases as development proceeds (though 

 not with absolute regularity). Thus the maximum exposure to 

 '21 consonant with subsequent normal development was 192 

 hours for embryos of six hours, 96 hours for embryos of one day, 

 72 hours for embryos of two to six days, 49 hours for embryos 

 of eight days, and 24 hours for embryos of 20 days. 



At these low temperatures development is stated to be com- 

 pletely arrested, though the heart never ceases beating, irregularly 

 and convulsively. The cooling process may be repeated over and 

 over again without altering the capacity for future growth and 

 differentiation, or reducing or increasing the maximum capacity 

 of endurance of cold. 



Malformations, as stated above, are of frequent occurrence, 

 but only in those cases in which the embryo has been exposed in 

 an early stage, during the first two or three days of incubation, 

 and only after long exposures. 



The medullary folds may remain unformed anteriorly, the 

 two halves of the heart may remain widely separate, the head 

 amnion fold may be absent and abnormal gill slits be formed ; 

 the heart and blood-vessels are often enormously distended, and 

 haemorrhages are frequent. Kaestner attributes these mon- 

 strosities not directly to the cold but to the pressure of the 

 blastoderm against the shell 5 for in the cooled eggs, owing to 

 some change in the specific gravity of the albumen or yolk, the 

 latter rises up ; if the egg is placed with the blunt end upper- 

 most, so that the embryo is pressed against the shell-membrane, 

 no monsters are produced. 



Mitrophanow is another observer who has utilized low tem- 

 peratures to cause malformations. High temperatures also give 



