ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 357 



tive mode, and tlie proliferating cells of the streak do nothing else than 

 divide and give origin to the mesoblastic bands. 



In the newt the blastopore appears to persist as the anus, but 

 examination of sections is still necessary to make this point certain. 



Gastrsea Theory.* — 0. Biitschli criticizes the theories of Hackel, 

 Lankester, and Metschnikoff, and puts forward some new views con- 

 cerning the origin of the Metazoa. 



The primitive form of the Metazoon is, according to this view, a 

 two-layered disk, which may be termed Siplacula, and which is supposed 

 to have originated from some such form as Gonium, a genus of 

 Volvocinese, consisting of a flat disk made up of many cells arranged 

 in one layer. It may readily be imagined that this one-layered cell- 

 aggregate soon undergoes differentiation, one side serving especially 

 for locomotion, and the other for nutrition ; and that these functions 

 become localized one in each layer of the placula. Some difficulties 

 arise about the way in which the gastrula takes its origin from this 

 two-layered placula, but the clue is to be found in the development 

 of certain nematodes : in CucuUanus and Rhahdonema the result of the 

 egg-cleavage is the formation of a two-layered embryo exactly similar 

 to the hypothetical placula ; in many other animals, e. g. Liimhricus, 

 vphich undergoes a developmental stage similar to that just described, 

 a segmental cavity is formed between the two layers and the embryo 

 becomes a blastula ; in other animals the segmentation cavity is more 

 highly developed and we get a typical blastula ; these facts indicate 

 that the blastula is not so ancient a developmental stage as the flat 

 two-layered embryo of the Nematodes. 



This two-layered placula reaches the gastrula form by a growing 

 together of the two ends, and the reason for this process of growth is 

 the hollowing out of the endoderm layer to form a receptacle for food, 

 which would evidently be of advantage to the animal ; also the 

 approximation of the endoderm cells would also enable the animal to 

 seize larger masses of food. This theory, therefore, renders in- 

 telligible the origin of the gastrula form, which the previous theories 

 do not. 



The marine organism Triclwplax adhcerens recently described by 

 F. E. Schultze, is a placula, and its discovery, Biitschli considers, 

 lends great support to the theory advocated in his paper. 



Changes of the Generative Products before Cleavage.f — M. 

 Nussbaiim describes his account of the changes undergone by the 

 generative products prior to the cleavage of the ovum as an essay on 

 heredity. He commences with an account of the copulation and de- 

 velopment of the genital products oi Ascaris megalocephala. Attention 

 is directed to the absolute similarity between the early stages of eitlier 

 sex, and it is pointed out that both the primitive ova and the sperma- 

 togonia increase by indirect division of the nucleus ; all stages of this 

 process may be observed. Later on, changes are to be observed in 

 both the nucleus and the cell-body ; granules deposited in the proto- 



* Morpti. Jahrbucli, ix. (1884) pp. 415-27. 



t Arch. f. Mikr. Anat., xxiii. (1883) pp. 155-21.3 (3 pis.). 



