ADDRESS. lxXXVli 



the ovum on the one hand, and the future embryo on the other, there is pre- 

 sented to us, by modern research, the interesting view that the blastoderm 

 consists, after completion of the segmenting process, of two layers of cells — an 

 outer or upper (usually composed of smaller, clearer, and more compact 

 nucleated cells), named ectoderm or ejpiblast, and an inner or lower (consisting 

 of cells which are somewhat larger, more opaque and granular, but also 

 nucleated), named endoderm or hypoblast. 



In the meroblastic ova, such as those of birds, the bilaminar blastoderm 

 is discoid and circumscribed as it lies on the yolk-surface, and only comes 

 to envelop the whole of the food-yolk in the progress of later development ; 

 while in the holoblastic ova, and more especially in mammals, the blastoderm 

 from the first extends over the whole surface of the yolk, and thus forms an 

 entire covering of the yolk known as the " vesicular blastoderm," the space 

 within being occupied by fluid. 



Huxley long ago presented the interesting view that these two layers are 

 essentially the same, in their morphological relations and histological structure, 

 as the double wall of the body in the simplest forms of animals above the 

 Protozoa. Haeckel has more recently followed out this view, supporting it by 

 his researches in the Calcareous Sponges, and has founded upon it his well- 

 known Gastrcea theory. According to this view all animals take their origin 

 from a form of Gastrula, or simple stomach-like cavity. In the lower tribes, 

 as in the instance of the common freshwater polype or Hydra, they proceed no 

 further than the Gaatrula stage, unless by mere enlargement and slight differ- 



into two, and each half moves towards the pole of the spindle on its own side, there being 

 radiated lines of protoplasm between the poles and the equatorial disk. 



The disk segments are the new nuclei, and the subsequent division of the cell takes 

 place in the intermediate space. 



Although these observers still differ in opinion upon some of the details of this process, 

 and especially as to the fate of the germinal vesicle, all of them seem to agree that there 

 are two pronuclei or distinct hyaline parts of the yolk-protoplasm, a superficial and a 

 deep one, engaged in the formation of the new nucleus ; and both Hertwig and Van Bene- 

 den are of opinion that the two'proceed from different productive elements. 



The radiated structure of the nuclei had been previously recognized by Fol and Hem- 

 ming, and further observed by Oellacher. 



1. Butschli's researches are published in the Nov. Act. Nat-Cur. 1873, and in the 

 Zeitschr. fiir wissensch. Zool. vol. xxv. 



2. Auerbach's observations iu his Organolog. Studien, 1874. 



3. Strasburger's observations in his memoir 'Ueber Zellbildung und Zelltheilung,' 

 Jena, 1875. 



4. Edward Van Beneden's researches, partly in his memoir "On the Composition and 

 Significance of the Egg," &c, presented to the Belgian Academy in 1868, and more parti- 

 cularly in the extremely interesting preliminary account of " Researches on the Develop- 

 ment of Mammalia," &c, 1875, and in a separate paper in the Jouru. of Microscopical 

 Science for April 1876. 



5. Oscar Hertwig's memoirs are contained in the Morpholog. Jahrbuch, 1875, and his 

 most interesting and novel observations in the same work, 1877. 



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