+886.) ` Embryology. 365 
à 2. THE DEVELOPMENT OF DENTALIUM.—M. Kowalevsky con- 
= ludes that the development of Dentalium has a good deal in 
common with that of the Lamellibranchs, the segmentation 
resembling that of Unio as described by Rabl, and Teredo as 
described by Hatschek. The segmentation is nearly regular, and 
eads to the formation of a hollow blastula and an invaginate gas- 
trula. The mesoderm`is derived from the inner wall of the invagi- 
nated side of the blastula, and the mesodermic cells are disposed 
symmetrically on either side of the median line. The shell-gland 
becomes defined very early on the dorsal aspect of the embryo, 
and as the blastopore travels forward, as in Patella, the area of the 
shell-gland, or mantle-organ, becomes greater, so that it gradually 
embraces the body of the embryo, especially over the region just 
behind the foot, leading to the development of the characteristic 
tubular shell. The resemblance of the larve of Dentalium to 
those of the Annelids is shown to be only a very superficial one. 
Three ciliary girdles encircle the anterior or cephalic pole of the 
larval body. The blastopore is wide at first, and persists as such 
much longer than in Patella; it is also elongated in the process 
of shifting towards the ventral, anterior aspect. A radular sac 
s developed on the inferior side of the cesophagus. The cepha- 
OE aC CS Peo ae eS Sree Ae 
is memoir, illustrated with eight well-executed plates, the 
figures being drawn from actual sections, is a very important con- 
bution to molluscan morphology, as nothing of equal value has 
appeared since the publication of the paper on Dentalium by H. 
Lacaze-Duthiers in 1857. 
2 
~This important paper by M. Kowalevsky discusses very fully 
h 
the 2 do not develop normally. The ova are enclosed by a 
_ nous covering, consisting of hexagonal plates which support — 
et externally, which vary in form in the various species. = 
“ae four first segmentation spheres are nearly equal ; each of : 
-Sse subdivide into two, giving rise to four upper and four lower = 
7 e polar globules rest near the center of the area 
"Etude sur PE ae ; i evsky, Ann. 
aca ‘PEmbryogenie du Dentale, memoire vil, par M. A. Kowalevsky, Ann. 
is T'Hist. Nat. de Marseille, Zool. Tom. 1, Seconde partie, 1882-1883. ee 
. : ibryogenie du Chiton polii (Philippi) avec quelques remarques sur le Pene at 
gg des autres Chitons, memoire v, par M. A. Kowalevsky, Ann. Mus: cee 
i Marseille, Zool., Tom. 1, second partie, 1883. 4to, pp. 46, pls. VII- a. 
