Bielefeld U, Becker W
Zoologisches Institut, Hamburg, Germany.
Int J Dev Biol. 1991 Jun;35(2):121-31.
During embryogenesis of the fresh water snail Biomphalaria glabrata (Say) (Pulmonata, Basommatophora) shell formation has been studied by light and electron microscopical techniques. The shell field invagination (SFI), the secretion of the first shell layers, the development of the shell-forming mantle edge gland and spindle formation have been investigated. During embryonic development at 28 degrees C environmental temperature, the shell field invaginates after 35 h. After 40 h the SFI is closed apically by cellular protrusions and scale-like precursors of the periostracum. The first electron translucent layer of the periostracum stems from electron dense vesicles of the cells which lie at the opening of the SFI. A second electron dense layer appears some hours afterwards. When the shell appears birefringent in the polarizing microscope (45 h of development) calcium can be detected in it using energy dispersive x-ray analysis. As calcification occurs the intercrystalline matrix appears under the periostracum and the SFI begins to open. In embryos of 60 h the mantle cavity appears at the left caudal side. When the mantle edge groove develops (65 h of development) lamellate units are added to the outer layer of the periostracum, but no distinct lamellar layer is formed in B. glabrata. In addition to the lamellar cell and the periostracum cell, a secretory cell can be observed in the developing groove. After 65 h of development, spindle formation starts and the shell begins to coil in a left hand spiral. After 5 days of development the embryos are ready to leave the egg capsules.
在淡水蜗牛光滑双脐螺（Say）（肺螺亚纲，基眼目）的胚胎发育过程中，已通过光学和电子显微镜技术研究了贝壳形成过程。研究了贝壳场内陷（SFI）、最初贝壳层的分泌、形成贝壳的外套膜边缘腺体的发育以及纺锤体形成。在28摄氏度环境温度下的胚胎发育过程中，贝壳场在35小时后内陷。40小时后，SFI顶端被细胞突起和角质层的鳞片状前体封闭。角质层的第一个电子半透明层源于位于SFI开口处细胞的电子致密囊泡。几小时后出现第二个电子致密层。当贝壳在偏光显微镜下呈现双折射时（发育45小时），使用能量色散X射线分析可在其中检测到钙。随着钙化发生，晶间基质出现在角质层下方，SFI开始打开。在60小时的胚胎中，外套腔出现在左尾侧。当外套膜边缘沟发育时（发育65小时），片状单元添加到角质层外层，但光滑双脐螺中未形成明显的片状层。除了片状细胞和角质层细胞外，在发育中的沟中还可观察到一个分泌细胞。发育65小时后，纺锤体形成开始，贝壳开始左旋。发育5天后，胚胎准备离开卵囊。
