396 NUCLEUS SAC OF THE AMNIOS. [BOOK i. 



Myrica, Alnus, Corylus, Quercus, and Juglans have been 

 named by Mirbel as plants in which the secundine is not 

 perceptible (Plate V. fig. 24). Its point is usually protruded 

 beyond the foramen of the primine. 



The nucleus (Plate V. fig. 22. b t 18, 19, 20. a, 24. d, 25. e) 

 is a pulpy conical mass, enclosed by the primine and secun- 

 dine, and often covered by them ; but frequently protruded 

 beyond the latter, and afterwards, at a subsequent period of 

 its growth, again covered by them. Sometimes its epidermis 

 is said to separate in the form of a third coating called the 

 tercine. It always contains an amniotic sac. 



The sac of the amnios is a cavity filled with fluid, lying in 

 the interior of the nucleus. It is the vesicula amnios of 

 Malpighi, the sac of the embryo or amniotic sac of others, the 

 additional membrane of Brown, the quintine of Mirbel. The 

 latter author originally described it thus : 



" At the centre of the tissue is organised, as in a womb, 

 the first rudiment of the quintine; it is a sort of delicate 

 intestine, which holds by one end to the summit of the 

 nucleus, and by the other end to the chalaza. The quintine 

 swells from top to bottom; it forces back on all sides the 

 tissue that surrounds it, and it often even invades the 

 place occupied by the quartine or the nucleus. A very 

 delicate thread, the suspensor* (hypostasis of Dutrochet), 

 descends from the summit of the ovule into the quintine, 

 and bears at its extremity a globule which is the nascent 

 embryo." 



The fluid matter contained within the nucleus is called the 

 liquor amnios, and is supposed to be what nourishes the 

 embryo during its growth. 



"There appears to be little," says Mr. Griffith, " definite 

 about the sac of the embryo, either in period of development, 

 situation, or structure : but ordinarily it may be recognised 



* It is this suspensor that Brown describes, in the ovule of Orchids, as a 

 thread consisting of a simple series of short cells, the lowermost joint or cell of 

 which is probably the original state of what afterwards, from enlargement and 

 deposit of granular matter, becomes the opaque speck, or rudiment of the future 

 embryo. (Observ. on the Organs, &c., of Orch. and Asclepiad. pp. 18, 19.) For 

 further information concerning the suspensor, see Mr. Griffith's observations in 

 the chapter on FERTILISATION in Book II. 



