318 yi. Ussow's Zoolo<ju'o-Kiid>rt/olo(jk'al Incestigativim. 



or (.•yliiulrical ci'lls (more elovatt.'<l at the upper, [)oiiite(l pole 

 ot" the egi::) torming the hhistoderin or upper genn-hunelhi 

 (h<^>rn-huneUa, sensory lameUa), on the secoiul [An/onnitta) 

 or third {LoUtjOy JS«^j)io/a) day of development a seconcl lamelhi 

 originates in the middle part {area ojmca) ot" the germinal 

 disk, hi/ transcerse dirifiioii of the upper germ-lamelJ<i | and this, 

 during the period of the apjtiarance of the organs, plays the 

 part of the middle geriu-lauuHa of the Vertcbrata, Annulosa, 

 Mollusea, &.C., and like that lanudla soon divides, in some of 

 the animals mentioned, into two layers, the dernio-museular 

 and the intestino-fibrous layers. 



From tlie lamella thus dividing, and indeed from the upj)er 

 first lamella wliieli becomes inverted on the two ojiposite 

 (ventral and dorsal) sides of the embryo, the young Cepha- 

 lopod is developed upon the broad hemispherical germinal 

 spot or disk, extending as far as the e(|uator of the c^ggj in 

 from 2o {Aryonauta) to 40 days {Loh'go). Tlu; lower part of 

 the germ, which in most of the species mentioned closes over 

 the obtuse pole of the Q^^g at the end of the first period, 

 l)econies the yelk-sac, composed of the upper lamella and the 

 dermo-muscular layer. 



Development commences in the central })art of the germinal 

 disk, and, indeed, by the apj)earance on the future dorsal sur- 

 face of the animal of an at first insignificant fmTOW, which 

 rather quickly acquires the form of a groove and subsequently 

 becomes converted into a perfectly closed tube. 



Simultaneously with the primitive groove appears the rudi- 

 mentary mantle, surrounding it and gradually groAving together 

 over it, whicli separates by constriction at first from the ventral 

 side, but afterwards and more slowly also from the dorsal side. 



Then there appear one after the other the eye-ovals, the 

 rudiment of the anterior part of the intestinal tract, the paired 

 rudiments of the branchitc, funnel, arms, and auditoiy^ organs, 

 and in the original solid anal tubercle the ])it-like depression, 

 whicli is afterwards converted into the ink-bag and the hinder 

 part of the intestinal canal (rectum). 



At a later i)eriod than the above-mentioned organs, the 

 central organs of circulation (auricles, ventricles, &.c.) and 

 those of the nervous system (the paired ganglia optica, cere- 

 bralia, pedalia, visceralia, buccalia, and stellata, and the un- 

 paired ganglion splanchnicum) make their appearance. 



All the organs appearing in the sequence just indicated are 

 develoi)ed from three different germ-lamcllai in one of two 

 ways : — either as a local thickening (excrescences and internal 

 thickenings) sometimes of the upper, sonu^timcs of one or 

 other layer of the middle lamella, or as an invagination or 



