[61] EMBRYOGRAPHY OF OSSEOUS FISnES. 515 



other part of the eiubrvonic skeletal frame- work of any form with which 

 I am acquainted. Perfectly cylindrical rods from their origins at the 

 sides of the notochord, they consist of a single row of hollow, discoidal, 

 apparently vacuolated cells, apposed by their flat surfaces. In their 

 vacuolated condition their component cells resemble the notochord, of 

 which they are evidently appendages, as already stated. The question 

 now arises, do they originate from a single line of solid cells along the 

 intermuscular septa ? Their condition as observed by us would appear 

 to favor such a view. The question is also raised as to their appendicular 

 relation to the notochord ; and what is the significance of their direct con- 

 uection with the sides of the chorda'? They appear like miniature lateral 

 repetitious of the chorda, but, unlike it, to be formed of but a single 

 linear row of vacuolated cells. Their points of insertion I have not 

 certainly determined to be intervertebral, but such they i^robably 

 are, since their courses follow the muscular septa. Hoffman has urged 

 a similar relation of the ribs and chorda in the embryos of other forms, 

 but I have not seen his paper on the subject. At these stages of devel- 

 opmeut of Gambusia the muscular plates were far advanced in devel- 

 opment and alread}^ presented the condition of a congeries of fibrillated, 

 cylindrical, or oval bundles of muscle fibers, and the distinction into 

 dorso-lateral and ventrolateral masses, with the horizontal lateral septa 

 of connective tissue developed between them. 



Xo observations on the development of the ribs of the young cod 

 were made, for the reason that no embryos of a sufficiently advanced 

 state of growth could be obtained. 



16. — Development of the skull. 



Upon this subject little can be said here, since, on account of the very 

 minute size of the embryo cod, I have not yet subjected the head of 

 the larva to a thorough examination by means of sections, the only 

 practicable method of studying this ])art of the skeleton of such a form. 

 Dissection is out of the question. Stated in general terms, my inves- 

 tigation has been conducted as follows: By carefully comijressiug the 

 embryos under a compressor of the proper form, the cartilaginous basis 

 of the chondrocranium may be revealed if a dilute solution of acetic 

 acid is used to develop the cell contours. I have also found that the 

 chondrocranium of larger forms of osseous fish larvse could be isolated 

 with tolerable success with the use of a weak solution of caustic potash, 

 which destroys the other soft parts, but does not so readily attack the 

 structure of the embryonic cartilage. 



As a result of such modes of investigation, it may, I believe, be stated 

 as generally true that the basicranial plate, peribrated by the pituitary 

 space and ensheathiug the anterior end of the notochord, is the first 

 portion of the true skeleton to be developed in osseous fishes, but the 

 skeletal axes of the branchial and hyomandibular arches develop their 

 rudimeiita about the same time. The branchial arches are formed froui 



