of the Fishery Board jor Scotland. 



115 



The Attachment of the Eggs. 



On being expelled from the ovary the eggs are received into the so-called 

 " incubatory chamber " formed by the curved abdomen. The perivi- 

 telline space rapidly develops in each egg. The abdomen is withdrawn 

 from the thorax, and the sixth abdominal joint and the telson are turned 

 upwards, giving a quadrant shape to a longitudinal section of the 

 abdomen, ab., fig. 14. The thorax forms the anterior end, the 

 abdomen the floor and posterior end of the chamber. The two sides are 

 formed by the exopodites, which by means of their plumose edges overlap 

 and prevent the eggs flowing out over the edge of the abdomen. The con- 

 dition is shown semi-diagrammatically in fig. 19. The eggs are apparently 

 extruded continuously until all are expelled. They then lie in a semi- 

 fluid mass in the " chamber," and embedded in the mass of eggs are the 

 endopodites with the flexible sharp-pointed hairs. The endopodites have, 

 independently of the exopodites, two distinct movements, of small extent, 

 one in an antero-posterior plane, viz., a. — a., fig. 19, and the other in an 

 oblique direction across the abdomen, indicated by the arrow, p. — p., 

 and p.' — p.' ; p. — p. referring to the endopodites of the right side, p.' — p.' 

 to the endopodites of the left side. This oblique motion belongs to the 

 distal parts of the jointed endopodites. The hairs reach every portion of 

 the receptacle. The continued double movement of the sharp slender 

 hairs through the mass of eggs confined in the incubatory chamber 

 results in the eggs being impaled and thickly skewered on to the hairs. 

 This condition is shown in fig. 1, which represents a hair taken from a 

 crab which had extruded its eggs only a short time, probably not more 

 than twenty-four hours, previously. In the drawing the perforations 

 in the zona are exaggerated. The hair avoids piercing the yolk, simply 

 passing through the zona into the perivitelline space, and then issuing 

 at a place near the point of entrance. Some dead eggs which were being 

 devoured by Nematodes and Acarinse were found on the hairs. How far 

 the death of the eggs was due to the accidental piercing of the yolk by 

 the hair, or to the unfavourable conditions under which the crab was 

 living at the time (viz., in confinement in a small hatching-box), is open 

 to question. The hair on striking and entering the zona will almost of 

 necessity force the egg to turn round in such a way as to bring the yolk- 

 sphere off the line of impact. The yolk-sphere would naturally tend to 

 keep at the lower pole of the egg. 



In a short time the zona collapses, and it becomes glued to the hair by 

 means of the perivitelline albuminous fluid. The stalk or pedicle is 

 formed by the adhesion together of the parts of the"' zona which meet. 

 This condition was found when the eggs were examined twelve days later. 

 Figs. 3a, 3b, 3c. An interval of that duration is, however, possibly not 

 necessary for this change to occur. The stalks vary in breadth, and they 

 are now more or less wrapped round the hair. All the crabs under 

 observation tbrew off their eggs shortly afterwards, but in a crab which 

 had spawned in a tank, and which was examined in January, the stalks 

 were now found to be rope-like in many cases. The stalks of the eggs 

 were also intertwined. The movement of the swimmerets, which is 

 probably continuous in order to afford aeration to the eggs, will, by 

 tending to throw the yolk-sphere as far away as possible from the point of 

 attachment, result in the formation of the long rope-like stalk, fig. 21. 



Some of the eggs are pierced by two hairs, and through this it happens 

 that the hairs are bunched together. This takes place not only with the 

 hairs of one row, but also with the hairs of adjacent rows. The grouping 

 of the hairs is, however, no doubt mainly due to the interlocking of the 

 eggs attached to different hairs. 



