WEYSSE. — BLASTODERMIC VESICLE OF SUS SCROPA. 309 



itself to the vessel, but offers nothing in evidence of this statement. 

 Ryder {'86), however, infers that the yolk does not become attached 

 until " after the vitellus has been covered by the blastoderm." During 

 last summer, when through the courtesy of Dr. Alexander Agassiz I 

 had the privilege of studying several weeks at the Newport Marine 

 Laboratory, I collected material of the cleavage stages of Batrachus 

 tau. This material was fixed without puncturing the egg membrane, 

 and a careful examination of it shows no trace of any attachment of 

 the yolk to the membrane, although the contents of the egg are every- 

 where in contact with it. It should further be remembered that the 

 contents are in a semi-fluid condition, and during the seven days men- 

 tioned abundant opportunity is furnished for a rotation of the yolk 

 within the egg membrane.* 



Before leaving this matter of the shape of the germinal disk, I wish 

 to refer to a young disk of the shrew, which Hubrecht ('90) has figured 

 (Plate XXXVII. Fig. 17) as elliptical, very much as I have described 

 the disks of the pig. From the position of the figure on the plate I am 

 left to infer that the shorter axis of the disk becomes the principal axis 

 of the embryo. In the later development he finds the disk ovate, but 

 he places the narrow end anterior and the broad end posterior, except 

 in one case (Plate XXXVII. Fig. 21), where the broad end is anterior, 

 just as I have placed it in my descriptions of the pig. This ovate out- 

 line, oriented thus, is certainly characteristic of slightly later stages, 

 and has been figured many times ; e. g. by Kolliker ('82) in the rabbit, 

 Duval ('89) in the chick, Keibel ('93) in the pig, etc. 



I now come to the last point which I wish to consider before taking 

 up the interpretation of the bridge. In the embryo represented by 

 Fig. 2, Plate I., and in all the following embryos figured on this plate, 

 there occurs a thickening of the entoderm, not only in the region of 

 the germinal disk, but also in a considerable area immediately sur- 

 rounding it. By a thickening I do not mean that the entodermal 

 cells have multiplied so as to be superimposed upon one another to 

 form a mass more than one layer of cells deep, but simply an increase 



* Very recently Morgan, (Experimental Studies on the Teleost Eggs, Pre- 

 liminary Communication, Anat. Anzeiger, Jahrg. VIII. pp. 803-814, 1893,) 

 working on the eggs of Ctenolabrus and Serranus, has arrived at the conclusion 

 that " there is no relation whatsoever between the cleavage planes of the egg and 

 the median plane of the adult body." Dr. Morgan bases this statement on the 

 observation of the axes of twenty-two eggs, and his method of determining the 

 position of the axes appears to be satisfactory. I cannot, however, enter upon a 

 fuller consideration of the subject here. 



