ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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demonstration of it in Leptoclinum Lacazii, where ova of two kinds may 
be found in one colony. 
The cases presented by Synascidians are not isolated in the Animal 
Kingdom ; differences of development in Aurelia aurita have been observed 
by Haeckel and by Schneider; Ophiothrix fragilis may deposit eggs 
which give rise to perfect or imperfect plutei or even to embryos which 
cannot swim and which go through a direct development. Both the 
author and Dr. Boas have shown that the size and number of the eggs, 
and the rapidity of metamorphoses in Palsemonetes varians are not the same 
in northern salt as in southern fresh waters. And, finally, Portschinski 
has discovered that Musca corvina has completely different eggs and 
larvae near St. Petersburg and in the south of Russia. These pheno- 
mena the author proposes to unite under the name of pcecilogony. 
Development of Distaplia magnilarva.* — Dr. M. v. Davidoff now 
gives an account of the general developmental history of the germinal 
layers of this compound Ascidian. The various parts of his subject 
are dealt with in the following order : — I. Segmentation and Gastrulation : 
1. The first three stages of segmentation ; 2. Further segmentation- 
stages as far as the development of the Plakula-form ; 3 Gastrulation 
and the formation of the fore-gut ; 4. Comparative survey of the gastru- 
lation of Ascidians — a. The relation of the gastrulation of Distaplia 
to that of other Ascidians ; b. Remarks on the development of the 
bilateral plan of structure in Ascidians ; c. On the relation of the axis of 
the gastrula to the body-axes in Ascidians ; and d. Some remarks on 
Rabl’s phylum of Vertebrates. II. Development of the mesoderm : 
1. In Distaplia; 2. In Clavillina Pissoana ; 3. In the simple Ascidians; 
4. Comparative remarks on the origin of the mesoderm in Ascidians. 
HI. On the formation of the gastric endoderm and of the chorda 
dorsalis: 1, 2, and 3 as in II. and 4. Comparative remarks. IV. On 
the development of the nervous system : 1. In Ascidians in general. 
2. Comparative remarks on the nervous system of the Ascidians. 
Y. On the separation of the tail-rudiment from the trunk. 
As far as the stage of four blastomeres segmentation in Distaplia is 
equal. The single asymmetrical phenomenon up till then observed is the 
excentric position of the cleavage-nucleus, which is retained unaltered in 
the nuclei of the first four cleavage-spheres. When the embryo becomes 
oval in form two endodermal cells remarkable for their small size may 
be seen at the hinder end ; these are not only characteristic of the 
hinder end, but are the first rudiments of the nerve-ring. 
After the completion of gastrulation the embryo remains for some 
time as a solid structure without any internal cavity. When a dorsal 
groove is formed its floor consists exclusively of endodermal cells. In 
simple Ascidians the early stages of development are of one type; 
cleavage is always equal and there is a more or less well-developed 
cleavage cavity which, sooner or later, becomes reduced to a cleft and, 
finally, completely disappears. The endoderm is formed by an invagina- 
nation and leads to the formation of an archenteron, the primitively wide 
orifice of which (blastopore) narrows from before backwards in such a 
way that it disappears latest posteriorly. In social Ascidians there are 
1891. 
Mittheil. Zool. Stat. Neapel, ix. (1891) pp. 533-651 (7 pis.). 
2 A 
