1896.] Embryobgy, 247 



ovum in such eggs as those of Jaera, and their direction, with conse- 

 quently the cleavage form, is due without other alternative entirely to 

 the constitutional peculiarity of the ovum. After carefully considering 

 the evidence presented by Jaera and similar centrolecithal eggs, the 

 assumption does not seem warranted, that they are any more removed 

 from the influences of external forces, than are holoblastic ova. It 

 may be true that it is difficult to understand how forces external to a 

 centrolecithal ovum may affect the spindles within it, hut many will 

 find the same difficulty in the case of holoblastic ova. Does the great 

 increase of yolk in a centrolecithal ovum remove the spindles from the 

 action of the external world ? I, for one, can not see that this neces- 

 sarily follows, and hence do not see that the condition of segmentation 

 in Jaera leaves us no escape from the conclusion that its cleavage form 

 is determined entirely by intrinsic conditions. 



Returning to the description of the embryo, it will be remembered 

 that the germ-layers are already distinguishable in a blastula stage on 

 the surface of the yolk in Jaera, and somewhat less distinctly in the 

 other forms. Now the blastoderm cells gradually concentrate towards 

 the ventral surface of the egg. This results in the mesendoderm and 

 vitellophag cells being crowded beneath the surface in the form of a 

 solid plug, and in the ectoderm of the ventral surface marking out a 

 somewhat triangular area, the base of which lies anteriorly while the 

 apex is posterior. This area is the Nauplius region. The rudiments 

 of the eyes are placed anteriorly at the angles of the base, the append- 

 ages appear later along the sides, while the blastoporic plug of mesendo- 

 dermal cells lies just under the posterior apical end. In a most 

 interesting discussion of the formation of the germs layers in the Crus- 

 tacea, the author concludes that the primitive Crustacea probably 

 passed through a blastula stage which was filled with yolk, and in 

 which a plug of cells migrated into the yolk to be later differentiated 

 into mesoderm and endoderm. This is the condition exhibited by the 

 Phyllopods (Samassa, 1893 and Bauer, 1892). Jaera, the Decapods 

 and especially Lucifer are examples of precocious differentiation of the 

 germ layers. The entire mesendoderm of Crustacea has a blastoporic 

 origin, and is not (except in Decapods, where there are secondary 

 phenomena) formed by delamination of extra-blastoporic region. The 

 under layer of the latter regions is formed by a migration of cells from 

 the blastoporic plug. In Armadiliidium this is especially well made 

 out. An interesting question is raised in regard to the mesenteron of 

 Astacus. Dr. McMurrich suggests the probability that the yolk- 

 pyramids do not form it, but eventually form u 



