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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



sidered in connection with what we know of the 

 movements which frequently precede the act of 

 division of the yolk-spheres, we seem in this 

 phenomenon to have made some near approach 

 to the observation of the direction in which the 

 molecular forces operating in organization may 

 be supposed to act. 1 



With respect to the nature of the blastoderm, 

 the organized cellular stratum resulting from seg- 

 mentation, and its relation to the previous condi- 

 tion of the ovum on the one hand, and the future 

 embryo on the other, there is presented to us, by 

 modern research, the interesting view that the 

 blastoderm consists, after completion of the seg- 

 menting process, of two layers of cells, an outer, 



1 The observations referred to above as to the division 

 of the nucleus are so novel, and of such deep interest, that 

 I am tempted to add here a short abstract of their more 

 important results from a very clear account given of them 

 by Dr. John Priestley, of Owens College, Manchester, in 

 the Journal of Microscopical Science for April, 1876. 



The researches now referred to are those of Auerbach, 

 Butschli, Strasburger, Hertwig, and Edward Van Beneden, 

 and the following- may be stated as the points in which 

 they mainly agree: 



The nucleus when about to divide elongates into a 

 spindle-shaped body, becomes irregular and indistinct, 

 acquires a granular disk or zone in the plane of its equa- 

 tor; this divides into two, and each half moves toward 

 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 subse- 

 quent 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 Beneden are of opinion that the two proceed 

 from different productive elements. 



The radiated structure of the nuclei had been pre- 

 viously recognized by Fol and Flemming, *and further 

 observed by Oellacher. 



1. ButscluTs researches are published in the Nov. Act. 

 Nat. Cur., 1ST3, and in the Zeitschr. fur wissensch. 

 Zool., vol. xxv. 



2. Auerbach's »bservations in his Organolog. Studien, 

 1874. 



3. Strasburgcr's observations in his memoir "TJeber 

 Zellbildung und Zelltheilung," Jena, 1S75. 



4. Edward Van Beneden'a researches, partly in his 

 memoir "On the Composition and Significance of the 

 Egg," etc , presented to the Belgian Academy in 1SG8, 

 and more particularly in the extremely interesting pre- 

 liminary account of " Researches on the Development of 

 Mammalia," etc., 1S75 ; and in a separate paper in the 

 Journal of Microscopical Science for April, 876. 



5. Oscar Hertwig's "Memoirs" are contained in the 

 Morpholog. Jahrbuoh, ls75. and his most interesting and 

 novel observations in the same work, 1S77. 



or upper, usually composed of smaller, clearer, 

 and more compact nucleated cells, named ecto- 

 derm, or epiblast, and an inner, or lower, con- 

 sisting of cells which are somewhat larger, more 

 opaque, and granular, but also nucleated, and 

 named endoderm, or hypoblast. 



In the meroblastic ova, such as those of birds, 

 the bilaminar blastoderm is discoid and circum- 

 scribed, 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 " vesic- 

 ular blastoderm ; " the space within being occu- 

 pied by fluid. 



Huxley long ago presented the interesting 

 view that these two layers are essentially the 

 same in their morphological relations and his- 

 tological structure with the double wall of the 

 body in the simplest forms of animals above the 

 protozoa ; and Haeckel has more recently fol- 

 lowed out this view, and supported 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. In the simpler 

 tribes, as in the instance of the common fresh- 

 water polyp or hydra, they proceed no further 

 than the gastrula stage, unless by mere enlarge- 

 ment and slight differentiation of the two primi- 

 tive layers of cell, representing the persistent 

 ectoderm and endoderm. 1 



If, pursuing this idea, we take a survey of 

 the whole animal kingdom in its long gradation 

 of increasing complexity of form and structure 

 from the simplest animal up to man himself, we 

 find that all the various modifications of organic 

 structure which present themselves are found, in 

 the history of the individual or ontological devel- 

 opment of the different members of the series, to 

 spring originally from two cellular laminae, ecto- 

 derm and endoderm, the component elements of 

 which may again be traced back to the first seg- 

 ment-sphere and primitive protoplasmic elements 

 of the ovum. 



Time does not admit of my conducting you 

 through the chain of observation and reasoning 

 by which Haeckel seeks to convince us of the 

 universal applicability of his theory, but I cannot 



1 At this place I will only refer to one of the most re- 

 cent of Haeckel's works, in which the views alluded to 

 above are fully exposed in a series of most interesting 

 memoirs, viz., "Studien zur Gastraea^Theorie," Jena, 1877. 



