MAY, 1891.] 



UNIVERSITY CIRCULARS. 



89 



folds secrete yolk which is passed into the egg. No egg pouch is present, 

 it having probably been absorbed. The nucleus occupies the position which 

 it has in all the mature eggs, viz., immediately beneath the ectoderm. 



The development of the egg pouches was not observed minutely, though 

 it is evident that they arise as mere diverticnla of the stem ; they have but 

 a single layer of ectoderm and of endoderm, which is continuous with that 

 of the stem. Haeckel represents the eggs of the egg pouch ("Polyovone 

 Gynophore ") as constituting a single layer next the supporting lamella, the 

 egiis being everywhere covered by a continuous layer of endoderm, and 

 separated from each other by endoderm cells. This work has shown that 

 the eggs are not covered with a layer of endoderm, but that they form a 

 part of the endoderm layer ; and if the eggs arc separated from each other 

 by cells, they are endoderm cells which have been torn from the lamella 

 by the growth of the egg. 



In Fig. 2, several stages in the development of some of the endoderm 

 cells into egg cells can be seen. It may be that the germ cells migrate 

 along the stem, as Weismann has observed in many cases, and finally take 

 a position in the layer of endoderm lining the egg pouches, but nothing 

 was observed to indicate that this does take place, and to all appearance the 

 cells which are destined to become egg cells are not different from the other 

 endoderm cells. So far as observed no eggs come to maturity in the egg 

 pouches; they are either passed on into the gonophore or are broken down 

 to nourish the egg which does pass into the gonophore. Usually but one 

 gonophore arises from an egg pouch, though in a few cases two or even 

 more arise from the same pouch. 



To sum up : The egg pouch must be regarded as a part of the stem where 

 the growth of the egg cells may take place while the gonophore is develop- 

 ing. As soon as the gonophore is formed one of the eggs, already quite 

 large, passes into it, where it lies between the ectoderm and endoderm of 

 the luanubrium. Then by the disintegration of the egg cells remaining in 

 the egg pouch, and by the formation of large endoderm folds which have a 

 secretory function, the egg is rapidly nourished and grows to a very large 

 size. The whole contrivance is to secure as rapid a development of the 

 sexual cells as possible, such as has been so excellently shown by Weismann 

 for very many Hydromedusae and Siphonophores. 



The fact that only female gonophores have been found on this "Albatross" 

 specimen, and the probability that the specimens examined by Haeckel 

 were females (since, as we have seen, the evidence that male gonophores 

 were present is very slight), suggests that the male may be widely different 

 in form. And it is highly probable that the female of Physalia is so differ- 

 ent from the male, which alone has been found, that if ever described, it 

 has probably been classed as a wholly different genus. 



EXPLANATION op FIGURES. 



All the figures represent longitudinal sections through the gonophores, and were drawn 

 with the camera lucida, under a magnifying power ol about 250 diameters. The space 

 occupied by the yolk, }', of the ovum, O, is left unshaded. 



REFERENCE LICTTEHS. 



A. Gonophore. n. Nucleolus. 



B. Egg Pouch. O. Ova. 



B N. Bell Nucleus. P. Pedicel. 



C Circular Canal. 7?. Rupture of Eudoderm and Lamella. 



EC. Ectoderm. 5. Stem orGonostyle. 



En. Endoderm. 5 EC. Subumbrellar Ectoderm. 



K En. Cathammal Eudoderm. T. Reunion of Ruptured Endoderm and 

 L. Supporting Lamella. Lamella. 



M EC. Manubrial Ectoderm. K Umbrella Cavity. 



M. En. Mauubrial Eudoderal. 1'. Yolk. 

 N. Egg Nucleus. 



Preliminary Note on the Embryology of Crepidula 

 fornicata and of Urosalpinx cinerea. By E. G. CONKLIN. 



Crepidula fornicata., 



The time and manner in which Crepidula lays its eggs has been described 

 by Dr. McMurrich. 1 



The cleavage follows the type found in Fusus, Planorbis, Neritina, etc. 

 'The first furrow is, with regard to the future embryo, a median longitudinal 

 one, and divides the ovum into exactly equal right and left halves. (The 



1 A contribution to the Embryology of the Marine Prosobranchs. Studies from Biol. 

 Lab., J. H. U., Vol. 3. 



method of determining the relation of the first furrow to the embryo cannot 

 be explained without several figures). The second furrow is at right angles 

 to the first and transverse to the long axis of the embryo, and it divides I he 

 egg into four equal inacronieres. ' Of these, two meet in the centre in a line 

 which KabP has called the " cross furrow ;" the other two are acute town rd 

 the centre and do not meet each other. By the position of the macromeres 

 with regard to the "cross furrow," the first and second cleavage furrows 

 may easily be distinguished, r. 17. , if the egg be viewed from the formative 

 pole and so that one of the cleavage furrows is in the line of vision, the 

 macromere to the right of this furrow and farthest from the observer will 

 be acute at its centre if the furrow in the line of vision be the second 

 cleavage furrow ; it will be obtuse, i. e., will meet the opposite macromeres 

 in the "cross furrow " if the furrow in the line of vision be the first cleav- 

 age furrow. Of course the reverse would hold true if the egg were viewed 

 from the vegetative pole. The examination of many hundreds of eggs has 

 shown that the position of the macromeres in relation to the "cross furrow " 

 and to the first and second cleavage planes is a constant one, and that the 

 first and second furrows may always be distinguished in the way mentioned. 

 The macromeres as distinguished from the micromeres do not again divide 

 until late in the course of segmentation, and as they do not change their 

 relative position it becomes very easy to orient all the future furrows and 

 cells with reference to the first two furrows. The polar bodies which are 

 at the centre of the ectodermal area also mark the centre of the dorsal sur- 

 face of the embryo, while the blastopore closes and the definitive mouth 

 appears almost directly opposite them. Until after the formation of the 

 blastopore the chief axis is the one drawn from the centre of the ectodermal 

 to the centre of the er.dodermal area; with regard to this axis the egg 

 is for some time radially symmetrical ; the chief axis of the embryo (the 

 antero-posterior) is at right angles to the chief axis of the egg. At an 

 early period there is a trace of a segmentation cavity, which however is 

 soon obliterated. The formation of micromeres proceeds with wonderful 

 regularity, following the law formulated by Rabl, 8 new micromeres always 

 being formed in fours either by being cut off from the four macromeres or 

 by the division of four micromeres already formed. When twenty-four 

 micromeres have been formed one of the macromeres, which later develop- 

 ments show to be on the posterior side of the ovum and to the left of the 

 median line, divides into a larger ventral and a smaller dorsal moiety. The 

 smaller cell, which, unlike the ectoderm cells, contains yolk, moves to the 

 right until it comes to lie at the posterior end of the median furrow between 

 the two posterior macromeres; it very soon divides into a right and a left 

 half, and at a later stage these two cells give rise to two mesoblastic bands 

 which extend forward over the right and left sides of the dorsal surface. 



Save for these mesoblasts the radial symmetry at the stage with 

 thirty-six micromeres is perfect. At this stage twelve of the micromeres 

 form a cross with three cells in each arm, the crossing of the arms being 

 exactly at the centre of the ectodermal area. Two of the arms lie in the 

 antero posterior axis, two in the transverse axis; one arm is anterior, one 

 posterior, one right and one left. In the next stage the radial symmetry is 

 no longer perfect, since three arms of the cross lengthen so as to contain 

 four cells each, while the posterior arm does not lengthen until later. How- 

 ever, with this exception, the radial symmetry is preserved until fifty-two 

 micromeres are formed ; at this stage the posterior arm lengthens by one 

 cell and the three other arms split longitudinally. At the same time each 

 of the macromeres, except the one mentioned above as giving rise to the 

 mesoblast, divides as did the later into a larger and a smaller moiety. The 

 three smaller cells take positions on the periphery of the egg in the furrows 

 separating the macromeres, one at each end of the transverse furrow and 

 one at the anterior end of the longitudinal furrow. 



In normal eggs there is not a trace of an imagination at the ectodermal 

 pole, such as has been described for Neritina and Fulga, though eggs which 

 were developing abnormally often showed such an invagination together 

 with other irregularities, e. g., the spreading apart of the macromeres and 

 the formation of large yolk containing cells at the periphery of the ecto- 

 dermal area instead of the small protoplasmic ectoderm cells which are 

 usually formed. 



The gastrula is formed by typical epibole. On all sides the ectoderm 



2 Carl Rabl, Ueber die Entwickluug der Tellerschnecke. Morph. .Takrbuch, lid. 6. 



3 IMC. oil. 



