310 



SCIENCE. 



[N. S. Vol XII. No. 295. 



shows, by means of curves, the number of 

 species of fossil elasmobranchs which are 

 known to have existed during each of the 

 geological periods. 



There is also presented a table which 

 contains lists of the genera of Elasmo- 

 branchs which occur in each of the geo- 

 logical periods. 



The portion of geological time occupied 

 by each of the families is discussed. 



Some conclusions are drawn bearing on 

 classification of the Elasmobranchs. 



The Lower Temperature Limits of Incubation 

 for the Egg of the Common Fowl: By 

 Chakles Lincoln Edwaeds, Trinity 

 College. 



Since the time of the Egyptians it has 

 been known that warmth is the chief factor 

 in incubation of eggs of birds. Modern 

 investigators have established 35 degrees 

 C. to 39 degrees C. as the normal tempera- 

 ture range. Eauber ('84) gave as the opti- 

 mum 38 degrees and minimum 26 degrees. 

 It is well known that cold, if not too in- 

 tense or too prolonged, will slow develop- 

 ment. 



Dareste gives 28 degrees C. as the physi- 

 ological zero for the hen's egg, below which, 

 of course, there is no development. 



Kaestner produced anomalies by inter- 

 rupting the normal development through 

 cooling the egg. 



"Warynski showed that yolk rises because 

 of change in specific gravity and sticks to the 

 vitelline membrane, thus producing arrest 

 of development and consequent monsters. 

 Fere ('94) established the ratio of devel- 

 opment at abnormal temperature to the 

 stage at normal of 38 degrees, as follows : 

 Temperature : 



34° 35° 36° 37° 38° 39° 40° 41° 

 Index of devel- 

 opment : 0.65 0.80 0.72 ? 1.00 1.06 1.25 1.51 

 In my experiments a Cyphers incubator 

 together with a calibrated thermometer di- 

 vided to one-fifths of a degree was used. 



Incubation 1. In 12 eggs incubated at 

 30.75 degrees C. for 7 days, 19 hours, chicks 

 reached an average of about one-half the 

 normal development. Over half of this 

 clutch of eggs developed hydropic vesicles 

 in the blastoderm. These arise from en- 

 lai'ged blood islands in the mesoderm, in 

 which the primitive corpuscles degenerate 

 and the space becomes filled with lymph. 



Incubation 2. In 6 eggs incubated at 

 29^ degrees C. for 5 days, 18 hours, the 

 ontogenetic stage was from the 16- to the 

 24-hour chick. The cephalic end of the 

 neural groove was trifid in one variate. 

 Lateral branches of primitive groove were 

 developed posteriorly. 



Incubation 3. In 12 eggs incubated at 

 28:^ degrees C. for 7 days the ontogenetic 

 stage was from a central area of undifferen- 

 tiated mesoderm to 27 hours. 



Incubation 4. In 10 eggs incubated at 

 27 degrees C. for 6 days, with the exception 

 of one uncertain anomaly, the greatest de- 

 velopment was represented by a primitive 

 streak 1.8 mm. long. Blastoderms vary 

 from 4.5 mm. to 8 mm. in diameter. 



Incubation 5. In 9 eggs incubated at 26 

 degrees C. for 7 days, 19 hours, showed a 

 primitive streak 1.3 mm. long as the great- 

 est development with the exception of one 

 case with open neural folds 1 mm. long. 

 Blastoderms vary from 4 mm. to 7 mm. 

 long. 



Incubation 6. In 8 eggs incubated at 

 25.5 degrees C. for 6 days there was a vari- 

 ation from no development to a primitive 

 streak and groove 1.7 mm. long. Blasto- 

 derms vary from 5 mm. to 5.5 mm. in 

 diameter. 



Incubation 7. 11 eggs at 25.5 degrees 

 C. for 7 days, 2 hours, 8 developed from a 

 central area of mesoderm cells to a primi- 

 tive streak 2 mm. long. Of the other three 

 one showed open neural folds and rudimen- 

 tary brain, one 22 mesodermic somites and 

 one was a 3-day chick. The last three may 



