Porifera. 



3. Embryology. 



For general conclusions see supra p 2 3. 



Noldoke finds in the free larva of Spongilla (fluviatilis and lacustris] a single 

 external layer of cubic cells, each with flagellum and proportionately large nucleus 

 with chromatiu network. These constitute the ectoderm: the rest of the larva 

 is the entoderm; it always contains a cavity under the Scheitelpol, the ento- 

 derm-cavity, visible in life, probably serving for the maintenance of equili- 

 brium ; an epithelial lining appears very early in the domed roof of the cavity, not 

 so regularly on the convex floor. The entoderm nucleus consists first of un- 

 differentiated cells containing cell -nuclei of doubtful chromatin structure, and 

 other bodies which the author agrees with other authors in calling yolk particles; 

 this identification is on general grounds , for no double stains or other methods 

 were found to certainly differentiate yolk from nuclear matter. As the yolk is 

 consumed these cells can be distinguished as Bildungszellen (= interme'diaires 

 of Delage), having reticular nuclei without nucleolus, and Amoboidzellen, 

 having sharply contoured nucleolus. Amoboidzellen diminish in number as the 

 growth of the sponge proceeds ; probably, as believed by Gotte and Maas , they 

 change into Bilduugszellen ; they can, however, form spicules or generative pro- 

 ducts. Under the ectoderm certain Bildungszellen become distinguished by their 

 flattened form as a continuous layer, the Epidermalzellen (= e'pidermiques of 

 Delage), enclosing the rest of the entoderm; some may still contain yolk-particles. 

 There are generally in the entoderm mass more or fewer spherical cavities, bounded 

 by Bildungszellen often bearing flagella, the flagellate chambers. Elodea 

 canadensis was found convenient for fixation, which takes place when the layer 

 of Epidermalzellen is completely developed (12 hrs. to 3 days). Adherence takes 

 place at a point near the Scheitelpol ; at the opposite pole creeping amoeboid Epi- 

 dermalzellen issue from the inner mass, and spread over the ectoderm as a trans- 

 parent cap ; permanent fixation is effected when this cap reaches and spreads over 

 the surface of support. As the flagella are retracted gaps appear between the 

 ectoderm cells; they become rounded, massing into irregular folds and agglome- 

 rations ; singly or in groups (sometimes the whole ectoderm together at one spot) 

 they pass into the interior of the larva. The Epidermalzellen form a definite flat 

 epithelium over all the outer surface. The ectoderm cells more and more lose 

 all coherence, until their easily identified nuclei are scattered throughout the larva. 

 Many amoeboid cells now contain , besides the nucleolate nucleus, bodies which 

 must be identified with the ectoderm nuclei. We cannot imagine new formation 

 of yolk ; and the more numerous such multinucleate cells become, the rarer are 

 free ectoderm-nuclei; we must suppose the ectoderm cells devoured by the amoeboid 

 cells. They do not escape again, but are digested and gradually disappear ; indeed 

 where there are heaped masses of ectoderm nuclei (as in the FuBzone) degeneration 

 seems to commence before the amoeboid cells can ingest them. Flagellate 

 chambers do not originate from the ectoderm cells, for (1) many chambers exist 

 before metamorphosis, (2) the ectoderm nuclei are seen to be digested, and (3) are 

 very different from nuclei of the collar cells. The internal changes of the larval 

 entoderm proceed independently of the epidermal layer ; where this is precocious 

 the larva fixes before any rudiments of chambers appear, and Delage's artificial 

 methods led him to believe this normal. The larval chambers are occasionally 

 connected with the eutoderm cavity, but this is secondary and without significance; 

 while the cap is spreading over the fixed larva, the entoderm cavity disappears 

 completely and its epithelium becomes again undifferentiated Bildungszellen. The 

 chambers are possibly prefigured in the young larva by spherical cell-masses and 



