ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 305 



Shell-structure and Pearl-making.* — A. Rubbell has made an 

 intimate study of the freshwater mussel, Margaritana margaritifera, as 

 regards its shell and its pearls. He calls attention to a fourth layer in 

 the shell, a clear layer which divides the nacreous layer into an outer and 

 an inner stratum. It is particularly well seen at the muscle-insertions^ 

 but it occurs in other parts of the shell and in the pearls. 



Rubbell is against the theory of the parasitic origin of pearls in the 

 freshwater mussel. He finds that they arise around particles of a yellow 

 substance which resembles periostracum. They originate in closed 

 single-layered sacs of epithelium which are constricted off from the 

 externarepithelium of the mantle, and like the mantle are able to secrete 

 all the layers of the shells. The pearls grow by the deposition of layer 

 after layer on their surface. The coalescence of several pearl-sacs leads 

 to the formation of curious pearl-conglomerates. What are called shell 

 pearls are formed in the mantle and become secondarily attached to the 

 shell. They are to be distinguished from shell-concretions which are 

 due to intruded foreign bodies and show no concentric lamination. 



Arthropoda. 

 a. Insecta. 



Tracheal Cells. f — -Th. Vieweger points out that this term is applied 

 to very different kinds of cells in insects, He describes a new type in 

 the caterpillars of Hypocrita jacobseae Linn., giant cells with a remarkable 

 juxtaposition of intracellular canaliculi and two distinct systems of 

 intracellular tracheae. There are relatively large tracheal culs-de-sac 

 in the cells and delicate rarausculi. The former end abruptly in cupolas ; 

 the ramasculi end in fine terminal twigs in the cytoplasm. The author 

 regards the intracellular canaliculi as endogenous formations arising 

 from the confluence of vacuoles. The tracheal culs-de-sac are probably 

 endoofenous also. But the ramusculi are due to an active invasion on 

 the part of trachea. 



Ocelli of Insects. J — R. Denioll and L. Scheuring have made a study 

 of the ocelli which many insects possess. They discuss their structure 

 and innervation, and they have made numerous experiments in the hope 

 of discovering their function. They seem to be auxiliary to the com- 

 pound eyes in faciliating the estimating of distance. 



Initial Genital Cells in Bombyx.§ — C. Vaney and A. Conte find 

 that the " polar cells," which become the primary reproiluctive ct-lls, 

 arise very early in Bombyx mori, below the peripheral zone of cells form- 

 ing the blastoderm. 1. In Clrironomus and Cecidomyia it has been 

 shown that the polar cells appear before there is any blastodermic dif- 

 ferentiation. 2. In Chrysomelids they appear along with the blastoderm. 

 3. In Orthoptera they appear metamerically below the germinal area. 

 The case of the silkmoth comes in between the second and third of 



these cases. 



* 



Zool. Jahrb., xxxii, (1911) pp. 287-36G (2 pis. and 60 figs.) 

 t Arch. Biol., xxvii. (1912) pp. 1-33 (2 pis. and 3 figs.). 

 X Zool. Jahrb.. xxxi. (1912) pp. 519-628 (23 figs.). 

 § O.K. Soc. Biol., hxi. (1912) pp. 712-13 (3 figs.). 



