1902.] CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. 45 



axis is probably the precursor of the first cleavage at right angles to 

 that axis. Morse ('73) also mentions the presence of a " distinct 

 granular membrane, the ovishell, which is formed while the eggs 

 are still in the perivisceral cavity." I have found this membrane in 

 all early stages (Figs, i-io), though I have not been able to recog- 

 nize it after about the twenty-cell stage. From the fact that it lies 

 close to the periphery of each cell, following all of its individual 

 curvatures (Figs. 2-10), I am convinced that it is not a distinct 

 membrane but only a peripheral layer of clear protoplasm (ecto- 

 plasmic layer of Harmer). There is no protoplasmic area in the 

 egg distinct from the yolk, though there is a slight aggregation of 

 protoplasm around the nuclei, but the entire egg is densely packed 

 with small yolk granules which jender it opaque. 



I have not attempted to study the nuclear phenomena of the 

 maturation and cleavage since the material is very unfavorable for 

 such work. Two polar bodies are formed, the first of which soon after 

 divides (Figs, i and 2). These polar bodies do not remain attached 

 to the egg after the sixteen-cell stage, and in some eggs they appear 

 to pass into the cleavage cavity, though in such cases it is difficult 

 to distinguish between polar bodies and small spherules which are 

 cut ofif from the inner ends of the cleavage cells, and which contain 

 protoplasm and yolk but no nuclei (Figs. 11 and 37). At the stage 

 when the gastrulation begins these spherules are found in consider- 

 able numbers in the cleavage cavity (Fig. 37) ; they disappear in 

 later stages. Similar spherules have been observed by Caldwell 

 ('85) in Phoronis.^ 



The first cleavage is meridional and divides the egg into two 

 slightly unequal blastomeres (Fig. 2) ; the second cleavage is also 

 approximately meridional and divides each of the blastomeres 

 equally ; as a result of this cleavage four blastomeres are formed, 

 two of which are somewhat smaller than the other two (Fig. 4). A 

 polar furrow is present (Figs. 4 and 7) which, taken in connection 

 with the overlapping of certain cells (Fig. 3), indicates that in 

 some eggs at least the cleavage is of a spiral type. The third 

 cleavage is equatorial and leads to the formation of eight blasto- 

 meres, all of which are nearly equal in size (Figs. 5, 6, 7) ; in some 

 eggs the four cells at the animal pole lie just above those at the 

 vegetal pole (Fig. 6) ; in others they have rotated through various 



1 Quite recently Ideka (1901) has fully described these spherules in Phoronis ; 

 he calls them pCasmic corpuscles. 



