1877.] 169 [Minot. 



layer of small protoplasmatic cells, usually called the blastoderm, 

 corresponds to the ectoderm. Nothing definite is known about the 

 yolk in Arthropods; if we judge from analogy it will be found to 

 belong to the entoderm ; at all events it forms large cells with much 

 nutritive matter. The supposed superficial segmentation is merely 

 apparent; the observations which sustained it are all evidently in- 

 complete, and in some cases have already been disproved (Spiders, 

 Ludwig; Crustacea, van Beneden, Bobretzky, Dohrn, etc.). 



From all these instances we conclude that the yolk undergoes in all 

 animals a total segmentation, during which the ceils of the ectoderm 

 divide faster and become smaller than the cells of the entoderm. Ac- 

 cording to Kowalewski's observations the Brachiopoda and Sagitta 

 are exceptions to this rule, — the only ones of importance Mr. 

 Minot had found. Renewed investigations may show that Kowalew- 

 ski's figures are not quite exact. 



All the known variations in the process of segmentation depend 

 merely upon: 1, the degree of difference ot size of the two sets of 

 cells; 2, the time when the difference appears; 3, the mode of devel- 

 opment, whether polar, or by delamination, either of which may or 

 may not be accompanied by axial infolding. In Gasteropods, Plana- 

 rians, Calcispongise, Gephyrea, Annelida, fish, birds, and Arthropods, 

 the difference is great and appears early. In Echinoderms, most 

 Coelenterates, some sponges, in Nematods, Amphibians, etc., it is less 

 marked and appears later. This stage of development is common to 

 all Blastozoa, and appears, according to van Beneden, even in the Di- 

 cynema. The term Diaderm may be used to designate it. Whether 

 the development is polar or peripheral, i. e., by delamination, the 

 space between the two divisions of the diaderm is that in which the 

 mesoderm develops. When delamination takes place the segmenta- 

 tion cavity is the entoderm cavity; when, on the other hand, the 

 segmentation is polar, the cleavage cavity is that in which the meso- 

 derm is formed. 



The two parts of the diaderm originate, as we have seen, in dif- 

 ferent portions of the egg. In some instances the finely granular 

 substance that generates the ectoderm collects at one pole, and the 

 segmentation becomes polar; in other cases it remains distributed 

 over the whole surface of the egg, and delamination results. Periph- 

 eral development is to be provisionally regarded as the primitive 

 mode, because it has been observed in three instances (Lamelli- 



