880 KECORD OF CURRENT RESEARCHES RELATING TO 



of a set of internal cells, covered by a layer of flatter ones, and both 

 of which owe their origin to the primitive ova and their investing 

 cells. The two sets continue to be develoj^ed equally, but the inner 

 ones undergo a differentiation, which, it may be noted, is very difficult 

 to observe as it is not a mori^hological one, but is merely dependent, 

 in its earlier stages, on change in position ; one cell becomes dis- 

 tinctly central and increases a good deal in size ; some of the peri- 

 pheral cells next increase in size ; the largest cell, which results from 

 those changes, is the future ovarian cell. Into their further history 

 it is impossible for us, in our limited space, to follow them, and we 

 must be content with stating that the large number of 1500 ova were 

 found by Lacaze-Duthiers in the uterine cavity. 



Passing on to the early stages in the development of the ovum we 

 find a striking difference between the fertilized and unfertilized eggs ; 

 in the former the oil-drops are arranged peripherally and form a circle 

 around the ovarian sphere, while in the latter the yolk is free from 

 oil-drops and is set a good deal towards one side of the egg, wliile the 

 oil-drops are limited to other parts. The first stage in fission was 

 not observed ; in the next, there were four cells of equal size, which, 

 like the unfertilized egg, consists of two parts, in one of which all the 

 oil-drops are collected, while the other, which contains the nticleus, 

 only consists of a finely granular protoidasm ; these two sets of struc- 

 tures may be known as vegetative and animal. In the next stage four 

 small animal cells are separated off, which immediately take up a 

 central position. The further stages of cleavage may be summed up 

 in a few words ; the animal pole having given rise to four blastomeres, 

 continues to divide and grow over the vegetative cells, which divide 

 more slowly. It is remarkable that this mode of cleavage has not 

 hitherto been observed in the Gephyrea, for Phascolosoma has been 

 found to exhibit equal cleavage of all the blastomeres and to form an 

 archigastrula ; nevertheless Trochus among the Mollusca, and Salma- 

 cina among the Annelids present just the same relations as Bonellia, 

 and similar relations have been observed in some marine Planaria and 

 Hirudinea ; to adopt the nomenclatiu'e of Haeckel we here have to do 

 with an amphigastrula. When the now subjacent macromeres begin to 

 increase they give rise to the enteric epithelium ; but the centre of 

 the embryo is still occupied by the four large macromeres, which are 

 remarkable for the possession of a large oil-drop, while the protoplasm 

 is reduced to a fairly large investing portion, containing the nucleus 

 on one side. When the endodermal and ectodermal cells become differ- 

 entiated an orifice — the blastopore — appears; and the ectodermal cells 

 soon pass into its inner margin. As to the mesoderm, it does not first 

 have the appearance of two large cells, as in Lumbricus (Hatschek, &c.), 

 but appears as a closed circular layer. 



At this point there are some gaps in Dr. Spengol's observa- 

 tions, and we pass on to the characters of the embryo and of the 

 free-swimming larva, in which we soon observe in the ectodermal 

 cells the characteristic green pigment, together with cilia which 

 appear on cells which are without pigment and which continue 

 to be so. A section taken at about this stage displays a mass of 



