OF ICTALURUS NEBULOSUS 



In Stage 31 a sluggish circulation is present 

 over the surface of the yolk sac, with the cells 

 of the communicating blood islands moving 

 slowly toward the venous end of the heart. 

 There are now in each otocyst two miniscule 

 otoliths, each composed of a number of minute 

 granules. During Stages 32 and 33 the opercu- 

 lum and gill arches differentiate, but circulation 

 through the gill arches does not appear until 

 late Stage 35 when it is established through the 

 first arch. 



At Stage 35 the head has extended forward, 

 carrying with it the operculum, resulting in a 

 gap between the operculum and the first gill 

 arch. During Stages 35 and 36 there is an open 

 communication through the pharynx from side 

 to side. This closes in Stage 37 by the backward 

 growth of the operculum. 



It is in Stage 37 that the first melanin pigment 

 is seen in the eye, sparsely scattered in the upper 

 posterior quadrant. Blood vessels form in the 

 eye in Stage 38 with an active circulation. Also 

 at this time a circulation is established through 

 the liver which is located along the posterior 

 margin of the left anterior cardinal vein lateral 

 to the body axis. 



In Stage 40, circulation is established in the 

 maxillary and lateral mandibular barbels, in the 

 pectoral fin and in the caudal fin, the latter by a 

 single vessel extending posteroventrally from 

 the aorta. Also in Stage 40, bile is present in the 

 gut at its anterior end and movements of the 

 lower jaw first appear. 



During the period of organodifferentiation 

 there is a progressive extension caudally of con- 

 tractility of the myotomes. At Stage 29 the tail 

 flexes laterally to the side of the head, at Stage 37 

 it sweeps over the dorsum of the head. Also in 

 Stage 37, the muscles of the anterior somites are 

 cross striated. In late Stage 41, progressive un- 



dulatory swimming occurs, the embryo com- 

 monly swimming on its side. Photokinetic re- 

 sponse to the "off" of light occurs first in Stage 

 42, with a well-defined negative phototactic 

 response demonstrable in Stage 44. 



Spontaneous hatching occurs in Stage 43, with 

 most of the embryos of an egg mass emerging in 

 three to four hours but with delay of some until 

 Stage 44. By the time Stage 44 is reached, there 

 is yellow bile in all but the terminal end of the 

 gut. 



The subsequent stages are marked by an in- 

 crease in size of the embryos and their ap- 

 pendages, development of the circulation into 

 the fins, and an extension of the pigment to 

 finally produce what is essentially the adult 

 pattern. The embryos are positively geotropic 

 until Stage 52, when some negative geotropism 

 appears. This becomes more marked as the em- 

 bryos progress into Stage 53. This negative geo- 

 tropism is probably correlated with the develop- 

 ment of the bilobed swim bladder which can be 

 demonstrated readily on dissection of fixed 

 embryos. Also in these late stages the embryos 

 apparently adapt to intensities of light which 

 earlier evoked a negative phototactic response. 



At 20 to 21° C. the development described 

 above through Stage 53 is completed in seven- 

 teen days. The table below gives the time 

 sequence of this development. 





Time Sequences, 







Development of Ictalurus nebulosus 







Stage Time 



Stage Time 



Stage 



Time 



1 1.2 hrs. 



25 2 days 



43 



8 days 



2 1.6 hrs. 



29 3 days 



46 



9 days 



4 3 hrs. 



34 4 days 



48 



10 days 



8 6 hrs. 



37 5 days 



50 



12 days 



14 9 hrs. 



39 6 days 



52 



15 days 



18 1 day 



40 7 days 



53 



17 days 



