OF ICTALURUS NEBULOSUS 



of such a size as to fit snugly in the top of the 

 battery jar. 



The embryos leave the nests and seek food 

 when the yolk has been absorbed. For a few 

 days they swim as a compact school in shallow 

 water guarded by the adults which swim about 

 below the school. After a few days the school 

 breaks up and the larvae disappear into deeper 

 water. 



To identify stages of the embryos, it is ad- 

 vantageous to examine them with transmitted 

 and with reflected light. Light transmitted 

 through the embryo from a white substage mir- 

 ror is especially effective in bringing out the 

 details of pigmentation. The developing blood 

 vessels and circulation are revealed best by the 

 use of a regular substage mirror. In lighting the 

 embryo from above, strong illumination, such 

 as that from a zirconium lamp, is required to 

 bring out the finer details. Also all available 

 features of the developing embryos, both mor- 

 phological and physiological, should be taken 

 ^ into consideration for accurate identification of 



stages. 



The embryos in the same egg mass, if properly 

 handled, show remarkable synchrony in their 

 development. However, minor but perceptible 

 variations do occur from mass to mass, possibly 

 because the development of the various features 

 of the embryo do not depend on a single com- 

 mon factor. Because of this, embryos will be 

 included in the same stage in spite of some such 

 minor variation from the pertinent illustration. 

 In the illustrations the embryos are magnified 

 1 7 times. 



Some of the special features of development 

 are described below. 



CLEAVAGE 



Stages 1 through 7 



Cleavage in the bullhead egg is meroblastic 



and meridional as is characteristic of the teleosts, 



the cleavages being limited to the blastodisc, not 



involving the yolk. The diameter of the early 



blastodisc is only about one-eighth the circum- 

 ference of the egg. The successive cleavages fol- 

 low each other at 35 to 40 minute intervals and 

 extend well through the disc. The blastomeres 

 formed at each cleavage are of equal size and 

 the cell divisions at any one cleavage are closely 

 synchronous. From the margins of the disc the 

 protoplasm thins out abruptly and extends 

 around the yolk as a thin continuous layer, as 

 in Fundalus. There is no defined periblastic 

 ridge such as occurs in Serranus (Wilson 1891). 

 Cleavage continues to be meridional in the bull- 

 head at least through the 32-cell stage and pos- 

 sibly through the 64-cell stage, but it is quite 

 probable that in this latter cleavage some of the 

 central cells divide horizontally. 



BLASTULA 



Stages 8 through 15 

 The subchorionic (perivitelline) space in the 

 bullhead egg is wide and the chorionic mem- 

 brane thin and yielding. This permits the for- 

 mation of a high blastula which is not seen in 

 those teleosts with narrow subchorionic spaces 

 such as Trutla, Fundulus, and Opsanus. The 

 early blastulae (Stages 8 and 9) very much re- 

 semble the morula stage of some of the inverte- 

 brates, the individual cells being large enough 

 to give the morula effect. The blastula remains 

 high (Stages 10 and 11) through additional cell 

 divisions and then gradually flattens out (Stages 

 12 to 15) with the formation of the blastocoele 

 which is apparent in the living egg (Stage 15). 

 The high blastula (Stage 1 1) has a diameter even 

 less than that of the one-cell stage, whereas the 

 flattened blastula (Stage 15) has a diameter more 

 than half again that of the one-cell stage. As the 

 blastula begins to flatten out in Stage 12, there 

 is the beginning formation of marginal periblast 

 cells, the process continuing through Stage 13, 

 resulting in the formation of roughly two rows 

 of such cells placed at a distance from the blasto- 

 derm. These periblast cells disappear later as 

 the blastoderm extends over the yolk. 



