Dr. W. Salensky on Hiickel’s Gastrea Theory. 19 
in a manner differing from the true Planula-form, but yet re- 
main perfectly homologous with the germ-lamelle of the 
Planula. These processes also appear to take place in the 
same way in the scorpion. 
Cases may, however, occur in which, after segmentation, a 
Planula-form is not at once produced. Most of these cases 
have been recently made known by the researches of Kowa- 
levsky and Mecznikoff in the Ascidia, Amphioxwus, Nemertina, 
&c. In these animals the egg passes through a so-called re- 
gular segmentation, and at the close of this becomes converted 
into a vesicle surrounded by uniform cells, which, to distinguish 
it from the Planula, may be named the “ Blastula.” The di- 
stinctions between the Planula and the Blastula are that the 
former already possesses two germ-lamelle, while the latter 
has still to form them. As the Planula-form in the Ccelen- 
terata issues from the egg and passes into free life, so also can 
the Blastula become free and swim about in the water, as is 
the case, for example, in the Nemertina (Mecznikoff, ‘ Mémoires 
de Acad. Imp. de St. Pétersb.’ tome xui.). In such a larval 
or developmental stage we can say nothing of either exoderm 
or entoderm. The two lamelle are still quite undifferentiated; 
this differentiation occurs somewhat later, and leads to a form 
which differs somewhat from the Planula-form. In some 
cases, before the differentiation into two germ-lamellz, this 
Blastula-torm may form a thickening at one point of its surface, 
to which the subsequent differentiation is confined, as seems to 
be the case, for example, in the Mammalia. Usually the dif- 
ferentiation commences in the Blastula by a portion of its cells 
beginning to distinguish themselves from the rest by some 
character. 
Let us commence our examination with the processes which 
indicate the differentiation in the Blastula of the Ascidia, as 
these have been best investigated. The first alteration in the 
Blastula consists in its becoming flattened on one side*. From 
Kowalevsky’s figures we see that at this stage (see Kowalevsky 
loc. ett. fig. 5 and Pl. V. fig. 4) the two germ-lamelle are 
already differentiated. The differentiation occurs in the same 
way in Lumbricus, where also the same flattening of the Blas- 
tula is the first thing that makes its appearance. I must spe- 
cially cite this first form of the differentiation of the germ- 
lamella, because in most cases in the above-mentioned animals 
the differentiation of the germ-lamelle has been confounded 
with the subsequent invagination; the latter, however, is a 
* Kowaleysky, “ Weitere Studien tiber die Entwickelung der einfachen 
Ascidien,” in Archiy fiir mikr. Anat. Bd. vii. p. 105. 
Q% 
