424 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



In Agekena, Locy (37) found the unsegmented egg to contain a central nucleus and protoplasm, 

 united by a fine network to a peripheral protoplasmic layer, the blastema. The nucleus divides, 

 and its products pass gradually into the blastema, which is used up in forming the blastoderm. 



Bruce (10) speaking first of Bobretyky's work, thus describes the formation of the blastoderm 

 in Lepidoptera: 



In the earliest stages he found four aracebiform cells in the yolk, situated in pairs at opposite ends of the egg. 

 Later the blastoderm isfornx^d byaranltiplication of these cells; according to Bobretyky it is not formed simultaneously 

 over the entire surface of the egg. Iiut is laid down first at one or more points on the surface. This type of 8egment.a- 

 tion can not strictly be called entolecithal, inasmuch as the cells are not, in the earliest stages of segment.ation, at 

 the surface inclosing the yolk. All the primitive nnditferenti.ated cells do not, according to Bobretyky, reach the 

 surface to form the blastoderm, but some remain ce.ntr,ally located as yolk cells after the formation of the blastoderm. 

 The earliest stages of segmentation observed iu Thyridopteryx showed several amoebiform cells in the yolk in each 

 cross section . 



In Thyridopteryx the blastoderm is first formed in a given area on the surface, afterward com- 

 pletely inclosing the egg, but in this particular closely related insects seem to differ. 



Heider's recent observations on HydropMlus (24) show that numerous nuclei which originate 

 from segmentation, do not reach the surface to enter into the " Keimhautblastem," but remain in 

 the yolk as yolk cells. 



The segmenting egg of Julus terrestris is described by Heathcote (19) as a syncytium. Seg- 

 mentation is at first central, and the resulting nuclei are united by strands of protoplasm. Upon 

 reaching the surface they spread themselves over it to form a blastoderm. The blastodermic cells 

 are united to each other by strings of protoplasm, and to the cells of the yolk. The entire egg is 

 thus iiervaded by a network of protoplasm. The yolk cells are regarded as entoblast, and give 

 rise to the "keel" or thickened blastodermic plate. This stage, characterized by a thickened 

 blastodermic disk, keel, or cumulus, probably corresponds to a similar stage which is met with in 

 Schizopods, Isopods, Myriapods, to the primitive cumulus in Arachnids and the ventral plate 

 which suffers invagination in some insects. Possibly it corresponds also to the invagination plus 

 the thickened ventral plate of Alpheus and Homarus. 



Possibly we have in Astacus and Homarus an approach to the mesoblastic type of segmenta 

 tion, such as is found in Mysis. This would be reached in Astacus if the protoplasm (which accord 

 ing Skimkewitch passes at once to the surface) should in the course of division build up a disk, 

 instead of diffusing itself over the egg. A similar result would be achieved in Homarus if the 

 belated cells should not reach the surface at all, and if those which are first to appear should not 

 diffuse over the egg, but segment to form a thickened plate. 



Balfour says, in speaking of forms like Peniijus: 



It is probable that not all the nuclei which result from the division of the first segmentation nucleus become con- 

 cerned in the formation of the snperficial blastoderm, but that some remain In the interior of the ovum to become the 

 nuclei of the yolk spheres. — (Comp. Embryology, Vol. I, p. 113.) 



This, I think, is an error, and that what is true of a number of forms, as Alpheus, Crangon, 

 Homarus, probably expresses the rule for the Decapods, that all the egg protoplasm enters into 

 the peripheral cell layer. Exceptions to the rule may, however, occur. 



The use of the term centrolecithal to express the relations of the protoplasm and the yolk in 

 the egg of Arthropods is not beyond criticism, but the strict application of a single term is clearly 

 impossible. The ground of any objection is sufficiently covered by Balfour, in emphasizing the 

 fact that it is the centrolecithal condition which is eventually attained. He says : 



As might be anticipated on the .analogy of the types already described, the concentration of the food yolk at the 

 centre of the ovum does not always take place before segmentation, but is sometimes deferred till even the later stages 

 of this ]>roces8. — (Comp. Embryology, Vol. I, p. 110.) 



In most cases the protoplasmic segmentation is at first central, or, as Kingsley points out, 

 ectolecithal, and then, after passing through the intermediate stage, it is finally centrolecithal. 



The question as to whether the products of the segmentation nucleus before the yolk is 

 involved, are to be regarded as independent celU was raised by Balfour, who, in reference to 

 Bobretyky's work on the embryology of insects, says : 



He regards the nuclei surrounded by protoplasm, which are produced by the primitive segmentation nucleus, .as 

 BO many distinct cells. These cells are supposed to move about freely in the yolk, which acts as a kind of intercel- 



