468 ZOOLOGY. 



ICHTHYOLOGY. 



The subjects of this "science" are separable under four classes: (1) 

 the Leptocardiaus, (2) the Myzonts or Marsipobranchiates, (3) the Se- 

 lachians, Sharks, Eays, and Chimserids, and (4) the true Fishes. 



CLASS OF LEPTOCAEDIANS. 



Hatschek (B.) StudieniiberEntwickelungcles Amphioxus. Arbeit. Zool. Inst. Wieu, 



V. 4, p. 1- 88, 9 pi. 

 Hoppe-Seyler (F.) IJber Amphioxuaund Cephalopodeu. Bericlitigung. Zool. Auz., 



4. Jalirg., p. 185-187. 

 Krukenberg (C. Fr. W.) Zur Kenntniss des cheniischen Baues von Ampliioxus lance- 



olatus und der Cephalopoden. Zool. Auz., 4. Jalirg. , p. 64-66, p. 263, 1881. 



Eml)ryology of Ampliioxus. 



In some respects the most important vertebrate type known is Am- 

 phioxus or Branchiostoma. Were it not for this form we would be left 

 in the dark as to the relationships of the great vertebrate with the other 

 branches of the animal kingdom and could only at best surmise, and 

 unperfectly, the truth in the case from the facts of the embryology of 

 the Marsipobranchiates and succeeding animals. Whatever light is 

 thrown on the history of Amphioxus, is consequently reflected on the 

 genealogy and history of all other vertebrates, including man himself. 

 A full knowledge of that form is therefore especially desirable. Numer- 

 ous naturalists have contributed to our information respecting the mor- 

 phology and histology of the adult stage, but few have done aught re- 

 specting the embryology. Years ago the eminent Eussian embryologist, 

 Kowalevsky, gave the first glimpses of the animal's earlier history, and 

 during the past year Dr. Hatschek has added to the stock of informa- 

 tion respecting its development, and confirmed, supplemented, and 

 sometimes modified Kowalevsky's account. The ova are generally quite 

 isolated. Oviposition seems to be dependent on the weather and even 

 time of day. The generative products are discharged through the 

 mouth, as Kowalevsky stated. The five fat-like bodies of the Eussian 

 author are regarded as yolk granules ; the spermatozoa would appear 

 to always enter at the vegetative pole. The progress of development is 

 described as it appears at five different periods. 



In the first period after the fecundation of the egg^ segmentation en- 

 sues. The cleavage was found to be unequal, the differences between 

 the two poles being well marked. There is a pause of about an hour 

 between the formation of the first and second groove. 



In the second, or " blastula-stage," the investing cells take on " an 

 epithelial character till there is formed a general outer layer, inclosing 

 a cavity. This simple epithelium forms the substratum for the later 

 developmental processes. All the essential organs are formed by fold- 

 ings or outgrowths from it. Bilateral symmetry is obvious at a very 



