296 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



a specimen preserved iu alcohol, in which that reagent has produced 

 the appearance of partial segra^utatiou of the j^elk. The germinal disk 

 is represented above with the surface of the cells flattened, probably by 

 the contraction of the egg membrane in a preservative fluid. The yelk 

 of teleosts, as in tlie egg of Elecate, for example, is sometimes apjjarently 

 di\ided iuto large cells, but such they are really not; they are merely 

 homogeneous masses of protoplasm involved in a diflerent kind of yelk 

 protoplasm. To sum up the matter, the comparatively full account of 

 the later development of Lepidosteus given by Balfour, and our lack of 

 knowledge in regard to the stages immediately' following the segmenta- 

 tion of the germinal disk and attending the formation of the cleavage 

 cavity and blastoderm, lead me to conclude that it is probable that it 

 will be found upon further investigation that the develo])uient of that 

 form is almost identical with that of the ordinary teleostean type. 



The development of the sturgeon when compared with the teleostean 

 differs from the latter mainly in the way in which the yelk is inclosed 

 by the intestine. This is certainly anomalous and not a little puzzling, 

 as it is the only vertebrate type yet known in which such an extraor- 

 dinary state of affairs has been shown to exist, and it is desirable that 

 this observation of Salensky's should be confirmed in our common 

 American species. The development of the germinal disk and blasto- 

 derm, from the account given of it by Kowalewsky, Owsjannikow, and 

 Wagner* does not differ essentially from that seen in the teleostean e,gg. 

 There is the same gradual envelopment and inclosure of the yelk by a 

 blastoderm with a thick rim, which makes the statements to the effect 

 that the segmentation is total appear at first to be founded upon doubt- 

 ful evidence 5 even Balfour admits that it "approaches the mesoblastic 

 type more nearly than the segmentation of the frog's f'.g'^.^'' The ])oint 

 where the blastopore closes appears to be the homologue of the anus 

 of Rusconi in the frog's 'd\^^, which is not the case in either the teleos- 

 tean or Lepidosteus. The three ichthyan types, however, appear to 

 agree i)retty closely in the formation of the segmental organs, muscle 

 segments, notochord, heart, and brain. In the blastoderm of the stur- 

 geon there is apparently a thick rim as in the teleostean, which is 

 mainly mesoblastic, and which in all probability contributes towards 

 the formation of the caudal i)late, and the posterior muscular seg- 

 ments, as in the latter. The segmentation cavity, according to the fig- 

 ures of Kowalewsky, Owsjannikow, and Wagner, ai)i)ears to be persist- 

 ent as in the teleost, and, if Salensky's representations are to be trusted, 

 it probably enters into the formation, not only of the body cavity, but 

 also that of the heart. 



The principal difference between the blastoderm of the typical teleos- 

 tean ovum and that of the Elasmobranch apj)ears to arise from the 

 mode in ^Ivhich the germinal disk continues .to spread over the yelk for 

 some time after the embryo has been formed and raised above the lat- 



""^ *Billlelm de rAciid. luip. desSci. dc- St. Petersburg, XIV. 318-3-^5. Ib70. 



