SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 



665 



4'6. Respiration of Fish Embryos 



A good deal of work has been done on the physiological and 

 morphological side of the respiration of fish embryos, but only little 

 on the physico-chemical side. As an example of the first type of 

 work, the experiments of PoHmanti might be mentioned. He gave 

 a good account of the first respiratory movements of the eggs of 

 Scyllium canicula, and described their association with the develop- 

 ment of the nervous system. Then a good deal is known about the 

 probable respiratory function of structures in some of the rarer 

 fishes; thus Ryder states that in the surf-perch, Ditrema laterale, 

 the caudal fin has hypertrophied blood-vessels in utero which serve 

 as respiratory portals for the embryo. This dermal vascularity dis- 

 appears before birth^. Compare this with the similar structures in 

 tropical land frog embryos described by Barbour. These questions 

 will be further discussed in the sections on the placenta. 



Quantitative observations began in 1896 with Bataillon, who 

 measured the carbon dioxide 

 evolved by the eggs of various 

 kinds of teleosteans, such as 

 the perch, the minnow {Pho- 

 nixus laevis), the vaudoise 

 {Luciscus jaculus), the rousse 

 [Luciscus rutilus) and the gud- 

 geon. He raised his minnow 

 embryos in a current of moist 

 air free from carbon dioxide, 

 and not in water, and found 

 that their development pro- 

 ceeded perfectly normally 

 in such conditions. He con- 

 cluded that there were two periods in development at which the 

 carbon dioxide given off per hour was rather low, one at a stage 

 just preceding the extension of the blastoderm over the yolk-sac, and 

 one after the occlusion of the blastopore. In later experiments he 

 actually placed the eggs in weak baryta, and found that they de- 



^ A remarkably interesting adaptation occurs in the case of the lung-fish, Lepidosiren 

 paradoxa. The eggs develop in burrows where the water contains no measurable dissolved 

 oxygen (Carter & Beadle) , but the male fish which guards the nest has long vascular 

 filaments on its pelvic fins during the breeding-season, and these may secrete oxygen into 

 the water around the eggs (Cunningham). 



Fig. 124. 



