M. Ussow’s Zoologico-Embryological Investigations. 215 
each other in the centre of each aggregation, so that a gradu- 
ally enlarging cavity is then produced, whilst the cells become 
elongated into a spindle-shape, and form rather thick mus- 
cular walls surrounding the cavity. 
It is only in the third period, for example in embryos of 
Sepia which are only one third or one fourth of the size of their 
yelk-sac, that there are, besides the above-mentioned consi- 
derably developed and already pulsating central organs of 
circulation, two so-called branchial hearts, situated at the 
broad base of the two multitubercular branchie. The walls 
of the aorta and of all the other subsequently appearing great 
arteries (e.g. of the optic ganglia), veins, and their diverti- 
cula (so-called kidneys) are developed from the cells of the 
middle lamella, which become elongated and arrange them- 
selves in rows. On the same day, behind each eye-oval, a 
spherical aggregation of cells of the middle germ-lamella sepa- 
rates; and these aggregations represent the rudiments of the 
optic ganglia. shall go into more detail with regard to these 
in describing the formation of the nervous system. 
At the end of the fourth day the cephalic lobes approach 
each other considerably, and the embryo rises above the nutri- 
tive vitellus, the walls of which, consisting only of a layer of 
cylindrical cells of the upper lamella and a layer of the middle 
lamella united to the former by means of thin, contractile 
protoplasmic processes, begin to contract rhythmically, by 
which the absorption of the nutritive vitellus is hastened. 
At the same time, the cells of the middle lamella (the dermo- 
muscular layer) surrounding the auditory vesicles, which are 
connected with the outer world by means of their peduncles, 
become converted into the envelopes of the latter. 
On the fifth and last day of the second period the thin ceso- 
phagus becomes deeper and extends nearly to the mantle, 
which at this time also rises somewhat on the back. In the 
anal pit, which has become somewhat deeper and acquired the 
appearance of a ceecal tube, a change takes place which is im- 
portant, inasmuch as it divides near the entrance into two 
tubes* :—an upper one, the rudiment of the 7n-sac, which has 
at first the form of a thin short tube enlarged at its ceecal ex- 
* This division is effected as follows :—Under the bottom of the anal 
pit, which is covered by two or three layers of the intestino-fibrous layer, 
a small excrescence is formed, which gradually raises the bottom of the 
pit nearly up to the entrance, and in this way, as by a septum, divides 
the pit into two tubes branching off at an acute angle. The bottom of 
the upper tube soon becomes wider; and at the same time the cells of 
its walls become considerably longer and thicker. In this way is pro- 
duced a sac furnished with a short efferent duct. The walls of the pit 
become higher and form the so-called anal lobes (Sepia, Sepiola). The 
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