M. Ussow's Zoologico-Emhryohgical Investigationa. 21o 



each otlier in tlie centre of cacli aggregation, so that a gradu- 

 ally enlarging cavity i.s then produced, whilst the cells become 

 elongated into a sj)indlc-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 branchiae. 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 genn-lamella sepa- 

 rates ; and these aggregations represent the rudiments of the 

 optic ganglia. I shall go into more detail with regai'd 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 aljsorption of the nutritive vitellus is hastened. 



At the same time, the cells of the middle lamella (the dermo- 

 muscular layer) surrounding the auditoiy 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 oeso- 

 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 cacal 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 ink-sac, which has 

 at first the form of a thin short tube enlarged at its caecal 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 bi-anching 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 wav 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 



15* 



