28 DEAN. [Vol. XI. 



but not to be seen, " the section merely passing through them 

 to one side." The origin of the auditory invaginations has been 

 well figured by Balfour and Parker: it is to be noted that 

 their appearance is about lo hours later than of the optic 

 vesicles. The pronephric duct in its earliest stages seems to 

 the writer more closely comparable with that of Elasmobranch 

 than earlier writers have inferred : the cell end, budded out of 

 the parietal middle layer was noted (in embryos of 1 1 somites) 

 as at first solid ^ later acquiring its lumen through the disasso- 

 ciation of cells; its irregular ridge-line projecting towards the 

 outer layer suggests closely a condition of Pristinrus, and its 

 early connection with the epidermis in the region of the fifth 

 somite is an additional character of comparative interest. The 

 pronephric duct occurs early in development, before even the 

 appearance of the lumen in the optic vesicle, or in the hinder 

 neuron, and before the constriction off of any part of the 

 gut, in strong contrast to its retarded development in Teleosts. 

 The history of the pronephros, as given by Balfour and Parker, 

 has not been followed in detail : this together with the discus- 

 sion of the inner germ layer might, perhaps, be better followed 

 in a study of the later development of the embryo. 



III. The Early Development of Acipenser. 



The egg of the Sturgeon when extruded is slaty-gray in 

 color : its general surface is pigmented and a rich mass of 

 pigment darkens the animal pole, often displayed as a cross, or 

 star-like marking (PI. Ill, Fig. 35). The germ-disc is already 

 clearly outlined, and its lighter color is made especially promi- 

 nent by an encircling darkly pigmented zone of the yolk. 



The ^%% membranes at this stage may be compared with 

 those of Lepidosteus as figured by Mark.^ The zona radiata 

 is similar in relative thickness, and the villous layer is clearly 

 distinguishable, although staining but lightly. The outermost 

 layer is distinctly separate from the others: as Salensky notes, 

 it stains readily (but irregularly) with haematox3din ; it is 

 many-layered and thick, often four times the thickness of the 



1 The present writer confirms the observations of Beard. 2 ^gj, 17. 



